This lens is spectacular!
Almost threedimensional.
See if you agree:
http://www.pbase.com/aggiedoggie51/image/90041272
Would not trade this lens for a heavier and bulky 2.8.
And I remember to play with ISO for a non-IS steady shot.
Great!

This is a great lens and ideal for anyone for whom its lighter weight outweighs the extra stop of the f/2.8. It may not equal the sharpness of a prime lens, but it is about as sharp of a zoom lens as one can find. The only real negatives are the high price for the tripod ring, and the very large hood. You cannot rotate a polarizer with the hood attached, and on a tripod, the hood can pickup wind and bring the weight too far forward.

Great lens very sharp where you need it most - wide open. Build quality superb. Focusing very fast and virtually silent. The weight and balance are good on 10D or 5D. Constant length means no dust being sucked in but makes the lens more visible. Drawbacks are that it is not a lens for covert use. I love this lens.

Very sharp, contrasty, no geometric distortions. Colors are excellent. Sharp even wide open. However it is extremely prone to flare; forget shooting into the sun. It's great for potraits; not as sharp as 85mm f/1.8, but still excellent. It's a great lens for landscape details, too. Tripod collar is an expensive option, so it might actually make more sense to go for an f/2.8 version if you shoot from a tripod/monopod a lot.

I need to be a voice of reason here. This lens is a nice optical performer wide open, and improves a bit when stopped down. The barrel is a hybrid of metal and plastic (NOT ALL METAL as some would have you believe). AF is fast, but not blindingly so. The focus ring turns smoothly but is 'dry' feeling like other USM lenses.

I acquired an EF 135 f/2.8 SF, and after that purchase found myself using the 70-200 f/4L less and less. The 135 somehow renders fine detail more crisply, but overall contrast is slightly lower. AF is just as fast if not faster than the 70-200 f/4L, and the lens is much more compact. It is also a stop faster which helps tremendously in all kinds of situations. The 70-200 also flares badly unlike the 135.

All in all, the 70-200 f/4L is a decent enough lens, but for my purposes cannot compete with the 135 SF. I sold the L and kept the prime.

For a last 6 month I've tried to find a good enough tele-zoom lens.
This was a really hard to make a right decision for me to buy a good quality lens due lack of budget. I asked myself: "Hey, man, what do you need? Do you really so need a good picture? Do you ready to spend extra $300-500 for that?". I'm an amateur and don't make money on shooting. So, i spent a lot of hours at the stores and make many shots with various lenses.
And when I saw that shots at home the answer was clear: "That one!".

Pros:
1. Superb build quality! Feels like it's solid metal. And looks like it's dustproof.
2. Fast and very quiet autofocus.
3. Clear and sharp images.
4. Colours is really great! Much better than EF 50 f1.8
5. Much lighter than f2.8 but heavy enough ;)
6. Great value for money! The best "L"-quality tele-zoom i've ever seen!

Cons:
1. No tripod mount in package
2. It's difficult to operate with lenshood when blend is attached
3. f4, but who cares ;) I'ts "junior EF 70-200 F2.8"
4. No rubber ring on EF-mount

P.S. I paid $700 in Moscow (Russia) wor this baby

Fantastic lens! Handles much better than the F2.8, and is a lot lighter to boot. Exceedlingly sharp for a zoom, just try an extreme blowup to see what I mean.