I'm a long-time Sonar user (since the ProAudio days).  I use it primarily for MIDI and soft synths.
 
My favorite version was 8.5.3.  X1 and X2 and X3 added "stuff" but didn't improve my productivity or enjoyment of using the program. 
 
Does the new version of Sonar significantly improve the UI for those of us who are mostly dealing with soft synths?  Does it increase the customizability?  Does it give us back some of the control that X1/2/3 took away from us?  Does it improve your overall workflow/efficiency?
 
Or does it just add more "stuff"?

I wish I could give you the answer you seek, but parts of your question are kinda loaded. "Does it give us back some of the control that X1/2/3 took away from us?" implies it is a given that we lost control to begin with. Not to argue that point, since you obviously feel that way.  For me each improved my productivity and it has nothing to do with what you refer to as "stuff".  Each successive version was faster  and more stable for me.  Granted, I don't use staff view, but i do use soft synths and MIDI.  For me 8.5.3 was good product, but contained a lot of things that were good in theory but horrible for me in actual use (beatscape and matrix view come to mind). I think SPLAT is better.  How much better depends on what your usage and workflow is. Perhaps wait for the demo to become available?

I don't know whether it is because it's a fairly fresh install for me however in the transition from X1 Producer to X4 Platinum on my system at least; I find it snappier and it easier to organise tracks. There seems to be less lag, although could be down to how I configured my drives this time and lack of internet security software. I also created a small batch file on my desktop to temporarily stop windowsupdate running in the background when I need to focus on things.
 
Personally I think what plug-in features are available are good however as I already own a few good convolution reverbs that wasn't my main purpose of getting the Platinum edition. It did include the majority of plug-ins I do use in X1 Producer, so that's what made my mind up about it.
 
I'm going to to rework a few pieces in this new platform and see if my time away from the DAW has been time well spent.
 
 

Yes, I think my questions are a bit loaded.  I'm a bit frustrated with how Sonar has gone in the past few years.
 
Like I said, I personally felt that 8.5.3 was the "best" version of Sonar, at least for me.  It had more features than I could use, and an intuitive UI, with good customizability, and it just felt right.  X1/2/3 feels awkward, like the designers were trying to deliberately veer away from the Windows-y-ness of the previous versions.  But it's the Windows-y-ness that I clicked with.
 
To me, X1/2/3 feels like a girl in one of those 80s movies who starts off as the "geeky" girl next door, but then gets a "makeover" and is supposed to look hot.  But doesn't.  She just looks like a generic barbie wannabe with a dated perm.  I much preferred the girl when she had glasses and a geeky laugh.  Know what I mean?
 
Sonar Platinum or X4 or X999 or whatever we're supposed to call it... it's basically a girl that you RENT, correct?  You pay $149 to upgrade to Sonar Platinum, and you can only use it for a year, before you are FORCED to pay for it again?  And again, and again?  For the rest of your life? 
 
I don't mind paying $10 a month for Photoshop, since I use it for work.  And as soon as I stop needing it for work, I can get rid of it.  With no qualms.  So subscription-based pricing works for me there.
 
However, music is a lifelong hobby.  So I will be continuing to make music no matter what.  But it's just a hobby.  It's impossible for me to say my hobby is worth $X per year.  And yet, if I'm reading the whole pricing thing correctly (and I might not be), Cakewalk wants to force me to pay $149 a year (for now; I'm sure that amount will increase over time once they have us "hooked"), and if I stop paying, I cannot use Sonar, so they hold my hobby at ransom?  They don't want to let me skip versions, or to stick with what's working for me?  Instead, they want to "force" me to "date" the chick with the barbie perm and bright red lipstick?  And each year she gets another makeover (high heels, fake boobs, nose ring) I'm just supposed to shut up and "go with the flow", to keep being her sugar daddy, to keep funding the makeovers, no matter how little I appreciate them?  No matter how much I preferred the girl next door?  Is that right???
 
Yeah, my questions are loaded.
 
I just wish someone would be able to say, "Hey, I know where you're coming from, because I felt the same way about X1/2/3, and I tell you, the new version of Sonar is much more your/our type.  She has her glasses back, her geekiness back, and is actually fun to be around.  That whole 80s makeover thing was just an awkward phase, and she's back to being truly cool again.  Trust me." 
 
Wishful thinking, I know. 
 
https://www.youtube.com/watch?v=oj20LKdg8-8

"Sonar Platinum or X4 or X999 or whatever we're supposed to call it... it's basically a girl that you RENT, correct?  You pay $149 to upgrade to Sonar Platinum, and you can only use it for a year, before you are FORCED to pay for it again?  And again, and again?  For the rest of your life?"
 
 
If you pay the full upgrade, you own it for life and get to keep everything.  You're not forced to rent it for life. 

Thanks for the clarification!  I was suspecting I was wrong about at least one thing! 

To some degree I know where you are coming from.
Color and toolbars customization - were more detailed in pre X series, and still is.
 
But I don't miss to a showstopper degree - even if I have some feature requests i in Ideas and features forum.
 
Colors - well, some stuff don't have a setting for you. You might have to indirectly get some things visible over clips by selecting clips for and background. Thinking of automation when track is not selected, I like to see that at all times. So I found that as a workaround.
 
Control bar is quite cool, and new Custom module is really good. A selection of command you put there, pretty much like a custom toolbar - but have a compact collapsed option to go away and hover mouse to let it expand, and as of now no icons for your chosen command.
 
Other things I appreciate are global bypass and pdc toggling in ControlBar. Really timesaver when need to do more recordings on a project that is close to final stage etc. You bypass pdc for record armed track kind of - and don't get the full project pdc while record monitoring.
 
Dim on solo I think is new to X-series as I remember it. Dim the overall mix unless on soloed tracks to hear what you need while recording.
 
Mic Recall - is really the one thing major feature that makes new Sonar shine brighter. Version of project with different plugins and settings, to use in various ways.
 
Member subscriptions are deeply misunderstood as you put it. You own whatever version you have after 12 months membership, downpayments by months or 12 months upfront. So it does not stop working - and you can re-enter any time to continue receiving updates and bugfixes.
 
The overall feel is really stable. My old lefthand reflex to save a project after each operation is pretty much gone. No worries in that department for me, anyway.
 
Still skeptic, rent it for a while and see what you think. It seems to be monthly releases with fixes and some new features in between as well.
 
Peek in Ideas and Features forum what users are missing - might give a hint too.

I have 8.5 installed on my old XP laptop and sometimes open it to help someone on the forum. That has become pointless because to much has changed. I too liked it "better" for a long time.I totally skipped X1 and 2. I had X1 LE as well as I downloaded the demo of X1. I stayed with 8.5 until I notice mostly positive comments on the forum about x3. The opposite was true of X1&2. So I upgraded to X3 Studio just when version e came out. 
 
Anyhow now when I open 8.5 about all I can say is 2 things I certainly miss about it.
The Colour is way brighter. X series is gloomy.
And I find the cut /copy /paste function 100% user friendly. I hate the new system with it's weirdness. I waste more time sorting out strange behaviour every time I try and edit an audio track.  
 
But over all the good outweighs the bad so we carry on like we do with all software upgrades.  I also miss features of Word 2004 but suck it up and carry on with Word 2011. 

I'm going to go out on a limb and say that if you don't like X3, odds are you are not going to think much of SPLAT.  We already agreed on the loaded question thing and I think there are certain things you already feel strongly about that are not likely to change.  The look of Sonar was changed because the look was one of the issues that reviewers often targeted about Sonar pre-x series.  I doubt they go back to anything looking like 8.5.  Hopefully they will refine things like color options, but the bottom line is short of an entirely from scratch UI redesign, the look is going to be built on top of the current rather than an attempt at regression on something they ditched 3 versions ago.
 
I hope I don't sound lacking in compassion.  I know what it is like to have a product i like change in a way i don't like.  I also know that it is very rare that companies do a 180 on that sort of thing, particularly when those things involve minority.  Even among  those who dislike the X series UI, most find the workflow and stability better.  There will always be as sect of those who long for the past.  I'm sure there is someone lurking who wishes the look and workflow was still like pro audio 9.  I can only offer my experience, as everyone else can.  Your situation seems like a catch-22 in the sense that the only way you will likely adapt to X series/SPLAT workflow that increases your productivity would be to spend enough time on it to see the benefits...which of course you would not do without first being convinced that they exist. In other words, it sounds like you need a verbal convincing for a problem that will take actual experience to resolve. Not saying that's a bad thing.  Perhaps if you live near a user, you could shadow that person long enough to make a better evaluation than just reading marketing material and anecdotes. 
 
I can see how you would think the improvements are mostly "stuff" because of the vast list of plugins and attention paid to such things but the actual experience is so different.  My work requirements mean I often find myself looking at the stuff quickly and then settling back into my regular workflow.  New features, unless they are blatant game changers or I luck out and something just works right from the beginning, tend to take a long time, if ever to integrate into what I do. For me simple things like being able to set up tracks to automatically take on the color assignment of the buses they are routed to is helpful for me and got used instantly. Other things like vocalsync may take awhile to actually become a go to tool.

PS- mix recall became useful day of installation.  I can sit down with an artist and show them vastly different mixes very rapidly.