just installed a new rear wheel, cassette and chain on my Trek.
went from the venerable (read decrepit and rebuilt too many times) original SunTour freewheel on a Sansin hub/Matrix rim wheel to a SRAM PG-730 cassette on a PowerTools hub/Sun CR18 rim wheel.
only thing i expect to miss is the chain (old-school Sachs), which i replaced with a KMC.
shifts pretty good, considering. the sprocket spacing doesn't appear to make much of a difference, probably since the cassette is a lot more sophisticated than the crude flat gears on my old freewheel.
looking forward to riding it, since it's a pretty big change in gearing too. the front is unchanged (duh), at 49/40/28. but the back has gone from 14/15/17/20/23/26/30 to 12/14/16/18/21/26/32. just from my initial test ride, i'm stoked about the new range - particularly the 32-tooth low.