Conceptually I liked X2. In practice it was an annoying arsed mofo to work with. There were lots of glitches and weirdness that slowed me down severely (which was very exasperating as a n00b still learning) and the new features which were well intentioned were not implemented as well as they could have been (half baked). All that and the lack of patches to fix some of these issues had me at the end of my rope with Sonar and Cakewalk in general.
Then came X3. I have not had a single problem with it (on the exact same system without any changes to it whatsoever) and the features that should have been awesome in X2 (but fell flat) sprang to life and become what they should have been and more.
There are also all the extra goodies that I did not think I "needed" or even "wanted" but after messing around with some of it there are things that I actually did need (or I at least find extremely useful) so that was just an extra bonus.
My point? If you like X2 then I see absolutely no reason why you won't love X3. It's just better.
I will say though $149 is the max price for a Producer to Producer upgrade. You should really wait for a special and snag it for $99 or some ridiculous freebies. We are nearing the end of the release cycle so the specials should be coming down the pipe very soon. We have already seen quite a few discounts.
So yeah, if you can wait then you could probably save some cash (if that is important to you). If you wait long enough you might even qualify for a free upgrade to X4 if they run their usual end of cycle promos. Then you'd have both.
Also give the trial a run through. Check out comping. That seems to be the one thing that some people have gotten weird about but personally I think it's great.
Cheers.