From a users perspective, not the feature list.
 
I went elsewhere at 8.5 and the upgrade sale is tempting me.  Seems like a no-brainer, but ...
 

Tricky to answer, totally new GUI which consequently changes the workflow but still pretty much in the same Cakewalk way.  If you go for the producer version the Pro channel will be a real revelation.  Download the trial give it a whirl. 

The overall functionally of 8.5 is very much there. The way you get to it has changed a lot. Most people have found X3 to be faster to do most things in then 8.5.  It is a real pleasure to use once you figure it out. I do believe its easier to learn than the old Sonar. 
 
There are many significant additions that will make things better overall as well. The biggest perhaps is Pro Channel. 
Then there is the new lanes with a very nice edit filter. Comping has been improved too. I'm not going to list all the new and changed things because that would be more a review. To me it is a great DAW. Far better than Sonar 8.5 was. 8.5 was no slouch.  
 
I would try the demo and see it for yourself. But you also need to read the manual. At first it may seem very unfamiliar but with question it is worth the effort to understand it.  
 
 
 

It's so subjective... the engine is better; the UI  is different. You may like it more; you may like it less. Either way, there's a learning curve. Your cheese has been moved and it'll take some effort to find it.
 
On the plus side, many bugs have been fixed in the compatibility and interoperability domains. Some VSTs that were glitchy in 8.5 are now solid.
 
I gripe at the (subjectively) stupid UI changes from the 8.5 family, but the fact I use X3 and have never gone back should be some kind of testimony.

rfreeze
From a users perspective, not the feature list.
 
I went elsewhere at 8.5 and the upgrade sale is tempting me.  Seems like a no-brainer, but ...

 
 
Too much.
 
I still use 8.5.
 
I never had stability problems. I run it at a 64 sample buffer and rarely have to step up to finish a mix. 
 
My VSTs work great. I buy from well seasoned plugin vendors and they play nice with the system.
 
There are some features unique to the Classic version of SONAR that no other DAW has. I like those features too much to walk away.
 
I paid for a couple versions of X but after trying them I uninstalled them. I bought a few other DAWs too.
 
I still like 8.5 as the all time favorite, and I have some new DAW favorites which I appreciate for their unique features.
 
 
 
 
 
The only way to know if you will like X+3+ is to try it, there are some great deals on upgrades right now, and I think there is a demo available too.
 
 

Go into it with eyes open...it's like if there was a new start-up company that unashamedly ripped off major parts of 8.5 but wrapped it in a (at least IMHO) better GUI, improved functionality/workflow, and placed more of an emphasis on stability. 
 
That's the good news. The bad news is that if you try to duplicate 8.5 workflow in X3 you will be frustrated. Often there is a different workflow that is much better (e.g., comping) but you need to learn it. I think the people who have had the greatest success transitioning from 8.5 to X3 have realized they need to "unlearn" 8.5 as well as learn X3.
 
Bottom line for me is you lose some time investing in the learning curve for X3. However, you more than make up for that time the more you use X3. I had 8.5 and X1 installed at the same time but never went back to 8.5; even with X1's birthing problems I still got projects done faster.

Comping is the reason I didn't buy X3.
 
My other DAWs have that sort of comping, and the advanced power of the Classic SONAR layers system is one of the tools that I would miss if I tried to use any of the other DAWs, including X3 exclusively.
 
You may satisfy yourself with thinking that comping is quicker with the "comping" tools but I find that you can not comp as precisely as you can with Layers so for me it has seemed like a step backwards.

It takes a little time to adjust from 8.5 to the X series but IMHO the effort is well worth it. I loved X1 from the outset and just gelled with it. I'm not a new user either, I started with V2.
 
The whole interface is much more sensibly laid out and as a result much faster to work in. Screensets alone are a revelation and there's so many improvements since 8.5 it'd be hard to list them all.
 
The best thing you can do is download the demo and go into it with an open mind and spend a little time re-learning the "new" ways. I'm sure if you're prepared to do that you'll find it time well spent.

mike_mccue
Comping is the reason I didn't buy X3.
 
My other DAWs have that sort of comping, and the advanced power of the Classic SONAR layers system is one of the tools that I would miss if I tried to use any of the other DAWs, including X3 exclusively.
 
You may satisfy yourself with thinking that comping is quicker with the "comping" tools but I find that you can not comp as precisely as you can with Layers so for me it has seemed like a step backwards.

If you are telling the truth then how could you know? You say you don't even have it and no longer use X1 or X2. 
 
How can you say anything about it?  You have a history of bashing the X series because you wouldn't adopt to it or couldn't. I think if you were to give your opinion on this you ought to know the program at least as well as you know 8.5.  Not having it sort of limits you as to the value of anything you say about it.  
 
Personally I always thought of layers as kind of clunky at best.    

>> how could you know? You say you don't even have it and no longer use X1 or X2.
 
I assume Mike tried the demo and watched the YouTube videos, like the rest of us.
 
Some of us prefer layers. It's very subjective.