After having used CS2 regularly for a while now, I can say this much:
It's about half as fast as CS. Still runs all right on my machine, but it's ridiculously slow if you plan to run it on an older machine. I have an AMD 64-bit 2800+ with a gig of RAM, and that makes it tolerable. I tried running it on a P4 1.8 /w 512 MB RAM ...ugh. Painful.
The RAM really helps more than anything else, I think. In fact, I purchased another 512 MB just after installing CS2 and there's been a significant difference after doing so.
A quote from their "New Features -At a Glance" pdf:
"Photoshop CS2 features memory management enhancement that breaks through the 2 GB RAM barrier. Now, devote up to approximately 3.5 Gigabytes of RAM to every Photoshop CS2 session."
Is it just me, or is that another way of saying, "Hello. Our application is a RAM hog. Buy more RAM, idiots!".
The new features are excellent, in my opinion.
Adobe Bridge is also kind of slow, but very helpful. The Batch renaming utility is very useful, since you now have the option of copying files to another folder and renaming them, rather than just renaming and moving or just renaming.
High Dynamic Range is another nice feature. I've found it more useful on landscape and interior shots than on, say, product shots. You do have to tweak it here and there to get the contrast you want, the auto settings always seem to suck somewhat.
The reduce noise feature is also quite good. It's not as good as Neat Image or Noise Ninja (so I hear, I haven't tried either of them), but it serves its purpose. I've always found it odd that Photoshop has had a filter all this while for adding noise with no filter or plug-in to remove it. I've never understood "Dust and Scratches", nor ever seen a good example of its use. High time they came up with it, if you ask me.
Incidentally, the Photomatix HDR was better than CS2s HDR. I downloaded the trial version. It's really good. Try it even if you don't have Photoshop.
I haven't experimented with red eye reduction (supposed to be better and now has a tool dedicated to it), but once I do, I'll post here with samples.
Smart sharpen pretty much sucks. USM is still the way to go as far as I'm concerned.
The new spot healing brush tool is quite useful for smaller irritants on photographs. Removing smaller CCD dust patches is easier. I mostly still use the patch tool for CCD dust, however. Just a personal preference.