I've played halfway through Katamari Damacy in PCSX2 already using gsdx9. There are some rogue polygon spikes here and there and sometimes you get stuck for no apparent reason and lose almost all the things you've rolled up. But it pretty much works.
I'm getting 20-40% speed in-game on a P4 3.4GHz with a Geforce 7600GT. A pretty cool thing I discovered as well is that when using Pete's SPU plug in, the music and sfx gets priority so that it doesn't skip and stays in perfect sync with the action.
I guess anyone with a C2D CPU and a reasonable video card should be fine, following customary 5-10 minutes of PCSX2 tweaking :-)
Also, this thread occasionally contains some interesting info/help:
Make sure to take your anti-lamer pills before you dive in though.