In reply to D0dge:
Funnily I bought the EF 70-300 USM IS and then exchanged it for a 70-200 f4 L (non IS).
I am not sre I made the right choice really although I love using the 70-200. I couldn't afford the IS version but bought ot on the basis that it's a reasonably fast lens (though not f2.8 fast!) you can up the ISO anyway. Optically it's lovely, build quality is great, and it's so smooth and fast.
The 70-300 I didn't really enjoy using greatly but I have to admit the IS was pretty astonishing in terms of what it acheieved. Also, it has a lot more reach than the 70-200, obviously. Build quality feels iffy but to be fair it wasn't bad. The main problem I found was that the IS absolutely gobbled up battery power and recharging became an issue. Which in hindsight was one of the reasons I went back from the IS to a non-IS lens. But I'm not sure that's a particularly valid reason.
I compared the two optically afterwards and the 70-200 is better in some respects, even on a PC screen. But the 70-300 gives great, sharp, result s.
To conclude, I love the 70-200 L but IS makes a hell of a difference. If I could afford the 70-200 L IS then I'd go for that in a shot.