Bit early days to tell for me.  I've not delved into the new features a lot yet besides the QuadCurve EQ zoom feature.
 
Seems to really not like Poulin's excellent range of amp sims very much (LePou's Lextac, Legion etc.)  Sometimes it will work, very often it'll just stop responding.  I'm using the 64-bit variants of those.  Shame as they're really good sims, especially in conjunction with his LeCab 2 IR cabinet emulator.

To follow up on the OP: I have X1 Producer.  Aside from the new instruments and plugins and considering only the new features from X1 to X3, please rate accordingly. I'm not trying to hijack your thread...
 
For example, as you can see in my avatar, I have a vdrums kit (td-20 with expansion card).  In every iteration of Sonar, I've found that the new drum plugins are fun to play with but I invariably end up back with my Vdrums for a lot of reasons.  I understand that the "samples" are better in some of the newer plugs, but the triggering and whatnot of the vdrums themselves, especially when considering the hi-hats, just works better for me.
 
I have similar positions about EQ and reverb and such.  I have a lot of plugins from all the older versions plus a few 3rd parties (none of those included in x3) and instruments I haven't touched in Dimension, Rapture LE, Sampletank 2.5 XL.  I have some great guitar effects via the PODfarm plugin and many other examples.
 
So, I'd like to know about the new features aside from the new plugins and instruments.

X3 is very stable here but then again every release I've had to date has been that way. Color customization in X3 is great. I'm really liking it.
 
Oh and I love the Nomad Factory Plugins. I don't know why they get such a bad rap but the ones I've tried so far seem to be quite good. I wasn't worried anyway having Pro Channel and the modules but glad to have the x-tras just the same.

I've used it for about 18 hrs since downloading it Sat. No problems so far with any of the plug ins, no crashes, love the new features, and well worth the 149 buck upgrade fee.
 
FWIW, I have an electronics consulting company and have several EXPENSIVE software applications that I need for my business. One of them is National Instruments Lab Windows CVI that sort of operates on the same principle. It is used to control my lab equipment via the program changing settings, taking measurements,etc. I even have to write code in C to make up a specific test program! The program sells for $3000.00 and upgrades are $1000 a pop!
Not really comparing apples to apples, since most of Nationals customers are corporations with deep pockets (I don't but need the program...) and they don't really have any competition.
 
I smile at the $149 upgrade fee for Sonar...
 
Walt
 
PS: Cakewalk Marketing. Don't read this post...