The only time I've experience lag issues with the Magic Mouse is when there is lots of traffic over the WLAN connection, like copying a large file over the network. Haven't checked if 10.6.3 does anything for this.
Otherwise it works great. I don't notice any lag in cursor movement and the multitouch gestures work well, though you must install either Steermouse MagicDriver (if you also want to fix the crappy OSX mouse acceleration) or BetterTouchTool/MagicPrefs if you want better gestures than what comes out of the box. Right click works as expected even with Apple's crap.
As far as ergonomics go, the Magic Mouse isn't perfect. I feel it could use a bit rounder edges, more height and also the two strips on the bottom should be made of a more slippery material because it makes quite a bit of noise on some surfaces.