I recently put a new Fox RP2.3 (air) on the 575. It's pretty sweet. With propedal on I have to try to make it bob - so far I've had 3 single-pivot induced gear skips in 90 or so miles of riding. It even handles out-of-the saddle cranking pretty well. With propedal off it's pretty plush. That said, I use about 90% of the travel (starting from 25% sag), so it's definitely not going to work for downhilling.
I also switched tires. I've been using 2.35 Kenda's for about a year now - Blue Groove front, Nevegal rear. I've really liked em, but needed a new pair and let the guys at the shop talk me into 2.5 Continental Diesels. The bad news - turns out a 575 doesn't really have the rear clearance for a 2.5 tire, even though it looks like it. I knocked the rim a couple millimeters off true in a rock garden on the Mormon Trail and that put the tire into the chainline. Plenty of clearance for the middle ring, though. So now I've got the 2.5 Diesel up front and a 2.35 Maxxis Minion in the back. While I like the Diesel (and I sure like the volume), I'm not yet sold that the volume is worth the rolling resistance when I'm climbing. I like the Minion perfectly well, but haven't yet ridden it enough to really differentiate it from the Nevegal.