I've been beating on X2 really hard since it came out. Quite a few crashes in the initial release, but I haven't had a single crash since the quick fix was installed. It's been rock solid for me.
As for comparing it to X1, I'd say there are things that are better and things that are "different and subjective". The last X1 patch totally fixed the issues I was having with it...so I don't ever have any problems with it.
The one thing I keep forgetting that drives me crazy is lasso-ing with right mouse button. I keep left clicking and dropping midi notes all over the place. Other than that though, I've been learning the new stuff pretty well.
Here's my advice to you, Amp. I would say wait until a REAL patch for X2 comes out. Though the quick fix has been wonderful for me, I have not yet installed X2 on my main recording pc's just yet and won't do so until a real patch comes out. That's not to say you will or won't have any issues, but me personally...I like to run it on a system to test it for a while and then wait for a major patch before I try it on my work horse machines. Just in case there is an issue, I can't afford to be down for an hour let alone have my system messed up. I haven't noticed any anomalies having X1 and X2 on the same machine....you know, where one messes up the other. But I'm still not taking any chances yet.
As far as the audio engine, in my experience is doesn't shut off nearly as often as X1 and previous versions of Sonar. It's rare for me to get a drop-out or have to re-enable the audio engine. I've also noticed about 5% cpu decrease. If I run a project in X1 and the same project in X2, it's anywhere from 3% to 5% less cpu in X2.
Some projects, X2 will use more cpu due to whatever modules I may be using in pro channel that won't be included in X1...so you have to weigh apples to apples when comparing the two. 64 bit X2 has been a bit weird for me as it seems to load up 32 bit plugs even though I have 64's of the same plugs installed. I had to create an entirely different plugin manager for my 64 bit plugs. I never had to do that with any version of Sonar. Usually, I can use the same plugin manager and when I click on something, it would bring in the 64 bit plug automatically if I'm using Sonar 64. I'm not seeing that in X2 though unless I use the new 64 plug manager I created...which I thought was strange.
But like I say, it has been working well for me. I still think it's in my best interest to wait for a real patch update though before I install on my real recording boxes so that's what I'm going to do. I think the upgrade price is more than fair though and would definitely be worth it for you to grab. It just depends on what your needs and feelings are. Good luck in whatever you decide.
-Danny