You're right...XD2 is polished and tends to work pretty damn well. It's tight and an all-around good desktop. The only catch is that you're completely confined to XD2. Ximian has their own modified libs for just about every Gnome library, so you can't mix Gnome libs and Ximian libs. You have to wait until Ximian has released their version to upgrade. For example, when Gnome 2.4 came out, I couldn't upgrade because I was using XD2 at the time, and they hadn't started rolling out packages based on 2.4 at the time. I was stuck using a beautifully polished desktop that was based on 2.2 and didn't have a lot of the cool stuff 2.4 did.
Basically, if you're going for a corporate desktop or an environment where you don't need to upgrade a whole lot, XD2 is a pretty good way to go. But if you're looking to stay on the bleeding edge and test out the latest and greatest gnome has to offer, don't go near XD2...stick with a stock gnome install.