I shot Pentax before joining the dark side (LOL) so these are my thoughts -
In body -
-Works on ALL lenses, it's nice to have a stabilized 50mm f/1.4
-Apparently "tuned" to the focal length of the lens
-Effective up to 300mm (my longest lens, I could shoot ~1/100s), but folks have good luck with the Bigma at 500mm
-You do have to wait a second after half-pressing to "prime" the anti-shake before taking the shot (you wait for a little hand to appear in the viewfinder), must repeat for every subsequent shot (at least in Single Shot focus, never really used AF-C) whereas Canon's stays "spinning" for some time after a shot
-Don't have to pay extra for each lens to have IS
-obviously tuned to each individual lens
-Stabilizes the viewfinder, VERY helpful with long lenses such as the 100-400.
-Runs continously between shots
-Makes IS lenses more expensive
So IDEALLY it would be great if Canon had in-body that could work with the non-IS lenses, and still allow you to use IS lenses if you have them. I would LOVE if my 85mm f/1.8 was stabilized. It can be tough trying to get fast enough shutter speeds with that lens in low light.