Hi Guys:
 
I'm running X2 on Windows 7 and am quite happy with it, but I'm very tempted to upgrade to X3.  So, talk me into it (or out of it as the case may be).  Is it a good step or not?

With Win 7 just do it, you're already wasting time :)
Don't forget to backup you system first!
Good luck...

I was happy with X2. I'm happier with X3
 
Melodyne is "the big deal" for many users, but for me the standout feature has been speed comping. I love it for vocals and guitar, saves me a ton of time. Also the Browser improvements have really added up over the past few versions, Addictive Drums is really good, VST3 support means I can run Padshop, and the Nomad Factory plug-ins - while they may seem redundant - have a character that's useful in a lot of situations. And there are ton of little things that make you go "is that new, or did I just miss it?"

X2 was unstable for me - so much so I never used it and continued to use X1.
X3 is solid as a rock and the melodyne integration is fabulous - and I have been a long time melodyne user, and have the full version of melodyne - even though I have had and used Melodyne quite a lot in the past on X1 etc, the new integration makes it so much easier to use and in my experience so far, far more stable.
Stability and Melodyne for me, make X3 a far superior option than X2
Nigel
 
 

X2 is the most stable and high powered  version of Sonar to date

Everyone has their own reasons for using X3 or deciding to use X3. As far as I'm concerned you will either get X3 or not even after you read all the comments here.
Either go for it or dont go for it. It's your life.

I am happy I moved from x2 to x3, it is more refined in so many ways. 
 
I would say if you are only an occasional user, stick with x2.  But if you are someone who needs to do a lot of mixing,  it's a must have upgrade as far as I am concerned.
 
And if you use pitch correction, the melodyne integration is just plain kick-button. Night and day from when I had X2. Absolutely love it.  So much quicker and easier. Almost seamless. And remember kids, Melodyne is not just about vocal correction.

Speed comping
ARA
VST3
Addictive Drums (Producer only)
QuadCurve Flyout EQ

'Nuff said.

If you use X2, I'm sure you will enjoy using X3. I think Craig hit the nail on the head, it's the little things that add up to a better user experience. I personally enjoy the Nomad plugins and Melodyne. Those two additions made it worth the upgrade on their own. The addition of easy to use track colours was an unexpected bonus. I haven't made much use of the comping features yet but it's something I'll probably get around too when the time is right. I use the pro channel EQ first, whenever I need an EQ, and the new expandable version with the frequency graph thingy, I have found, is an invaluable ear aid.
 

Don't forget BlueTubes... really wonderful stuff!  worth the price alone... 
all in all, a no brainer...