For: Rich colours, image quality is really in a league of it's own, gorgeous bokeh at F2.8 through 70-200mm, fast and accurate focus in low light, tough well shaped lens hood, lense does not move forward or back when using zoom, image stabilisation means you can hand hold as low as 1/50th and get sharp images.
Against: Attracts attention - it's not a discreet lens, expensive, minimum focus distance isn't that close which limits its usage in smaller areas.
The image quality is just in another league to any other lens I've used before. Every photo I took with this lens just had a 3D quality to it. The images literally jump off the screen.
At 200mm the F2.8 bokeh is simply breathtaking and I've read many people saying this lens isn't as sharp as prime lenses.
If I could add anything I would agree - prime lenses are sharper BUT this lens still produces sharp images and for me the colours are much more vibrant than the primes I've used and again the image quality is on another level.
Like most L-glass the lense hood is tough and fits tightly and unlike the 24-70mm the front of the lens does not move so lens flare is rarely if ever an issue.
Another good thing is you can throw on a 1.4x converter which in no way affects the performance and gives you a much longer focal range should you need it. The bokeh at F5.6 still looks great!
So to wrap things up, If you've been saving a while for a fantastic walkaround lens with a little more reach which produces breathtaking images be it portraits, scenery and anything in between this lens is well worth a look.
As with all lenses, if you can then try before you buy!

Canon 70-200mm F2.8 USM L IS, ISO 1600, F2.8, 1/50th

Canon EOS-1D Mark III @ 153mm, F2.8, ISO: 1600, 1/50th
Canon 70-200mm F2.8 USM L IS, ISO 200, F2.8, 1/3200th

Canon EOS 20D @ 200mm, F2.8, ISO: 200, 1/3200th


