I installed the "Dapper Drake" testing release on a spare computer the other day, and I have to say, DAMN I'M IMPRESSED! It's a lot faster, at least it felt that way, I haven't done any benchmarks or anything like that. The boot-time is much faster. Gnome has had major updates, and for the most part, I like them. I think the logout/shutdown/reboot window is a little too big, but that's really my only nitpick so far. I didn't get to try this XGL/Compiz thing the whole linux world is abuzz about, because the hardware in my test-box isn't the greatest:

Pentium III 1ghz
512MB PC-133 SDRAM
40GB Maxtor HDD
ATI Radeon 7000 64MB

I'll post more as I have more time to try things out.