It's incredibly sharp, edge to edge and across the zoom range with good colour and contrast and a mind boggling 4 stop IS. Lab tests I've read indicate that the performance is only very slightly worse than equivalent Canon 'L' primes, and that's quite an achievement in a zoom.
It's also much smaller lighter when compared with the f2.8L, and not that much heavier than the non IS version. I can carry it around all day, where'd I'd rather not lug the f2.8, and that's really what it's all about.
Other than the fact that it only goes to 200mm and is white I really can't think of a bad thing to say about it. I can't be the only person that would be prepared to pay a little more for a black 'L' lens...
The lens is really quite long (not much shorter than an un-zoomed 100-400L), but *doesn't* extend (or rotate) when zoomed and the zoom ring doesn't have to be turned too far to go from 70-200. You can still manually focus with AF switched on and focusing is as quick and quiet as you'd expect from an 'L' series lens, so it copes really well with moving targets.
If you have the money, can settle for 200mm top focal length, want to take a lens out and about and don't mind white, it's almost a no-brainer.