Pros: Super-sharp images, Superior build quality, Fast auto-focus, image stabilization, built in lens hood, size and weight
Cons: not weather sealed
I've been using this lens for 4 years now to photograph surfing, air shows, wildlife, etc. It is tack sharp and contrasty wide open at f/4. I can and often do crop the center half out of images shot with this lens and the resulting prints are still sharp. I've gone so far as to crop the center 1/4th out of some air show shots and, as best as I can tell, the limitation is not the lens sharpness, but the sensor resolution in my DSLR.
IS works amazingly well, in my experience better than the 2 stops Canon claims. Trust me when I say that IS helps after a long day of shooting when tired arms would otherwise compromise even normally safe shutter speeds.
Bokeh and color are excellent, as is the build quality. My only complaint is that Canon did not weather seal this model, though it's certainly more resistant to weather than the typical 'telescoping' consumer zoom. AF is not quite as fast as my 70-200, but fast enough for every subject I've put it against.
I can't say enough good things about this lens. It is one of my most productive lenses.