For me I hope I never have to return to X3e. There are way to many new features I use daily that I cannot see doing with out now. It totally depends on how you use the software. 
They have added a LOT since X3e. All for the better. 
You can ceratinly install the latest Splat and keep x3e too. I had both up until last week when I re built. 
My list of features I use all the time:
Drag and drop a Audio BAss track to create a midi track. 
Drum replacer to convert an audio drum track to midi. 
The new PVR with the ability to view all midi tracks at the same time and jump around as the song plays. 
This one feature I can not think of loosing and probably the main reason I am staying with Sonar. 
 
Not sure if X3 had the smart tool but that is huge for me and the deal breaker with all other DAWS I demoed. 
The new lenses and screenset options make it easy to set up a workspace that stay's put from project to project. 
The Pro Channel.. x3?
Improvments to Rapture and Dim Pro. 
And probably a dozen other things that have been updated. 
 

Cactus Music
At at what point has W10 update ever stopped Sonar from running? 
I didn't say it doesn't cause problems, I am abandoning it because it does cause problems, but it has never actually trashed Sonar for me,, just my drivers and settings. 
So I agree to disagree....

It has caused problems.  Period.  And if it did it before, it can do it again. 
 
You keep bringing up your personal experience, but how is it relevant if you weren't affected by the issues?  You ask, "At what point has W10 update ever stopped Sonar from running?"  According to the linked article, that issue caused Sonar to crash in a variety of situations.  Isn't crashing the same as "not running"? 
 
Yes, issues like these were much less of a concern with an active programming staff squashing the bugs -- but CW's development and support is done.  It is simply irresponsible to keep telling people that there hasn't been, or likely will ever be in the future, any Win 10-related problems affecting Sonar.  

Cactus Music
Improvments to Rapture and Dim Pro.

These synths have not changed since 2010. Only changes made were to the installers so they could run in the Command Center.

I think like I say , I do not like W10,, I myself had a whole slew of crashes after the August W10 update. 
But those are at this point on my W10 computer very rare. I'll see if my W8.1 machine does better. I think it will but then? 
The point will be that Sonar will continue to run under W10 but with pittfalls. I fail to see that MS would wish to develop an OS that made everyones software stop running. Why would anybody use that version?  

pdarg
Thanks to all for their responses.
I guess I should also ask this:
I purchased the lifetime Sonar Platinum upgrade - but never installed it.
Is there any reason I should use Platinum if I am happy with X3? Any big advantages to using the newer version?
Thanks again.

You can have both programs installed on a Windows 10 system. I just never uninstalled X3 when Platinum came along, although I'd open it from time to time for testing compatibility and such.
 
IMHO Platinum is much better than X3 due to, if nothing else, the speed improvements. However the LP EQ, LP MB, and Adaptive Limiter are also a pretty big deal, and TH3 Cakewalk Edition is better than TH2.
 
Set a system restore point and go for it! I doubt if you'll run into any issues.
 
 

The only problem I know of when installing the current Platinum next to older versions of SONAR is the current Plug-in Manager is not backward compatible. It was broken several releases back. It is possible to run multiple Plug-in Managers by saving/renaming the old version prior to installing Platinum and modifying the registry for older versions of SONAR to see the old Plug-in Manager.

Anderton
TH3 Cakewalk Edition is better than TH2.

Yes, especially since Overloud was kind enough to unlock the plug-in.

Good to know, I never ran the PIM in X3 after installing Platinum because I liked Platinum so much better. It's where I did all my work.

The biggest problem is custom layout management. Pretty sure the exclude list and VST properties still work.