Totally agree, have used one for about 20 minutes on my 1D and it's awesome.
One of Sigma's best lenses, along with the 100-300mm f4, 24-70mm f2.8 and 50-500mm. Probably wouldn't consider using it on the 300 or 350D without a grip, i would imagine it to feel very strange!
The only thing that's stopping me from buying is:
A: I don't have the money for it.
B: The Canon 70-200mm f2.8 IS L is the only 70-200mm lens I need every buy.... even if it is twice the cost, i would never need to upgrade.
C: B is cancelled out by A