Hmmm......Seems like everyone's replying about lots of lenses the OP didn't ask about. So I'll give my 2-cents worth about the two lenses actually in question. I owned the 70-300 IS, then got a 70-200 f4L (non-IS), then went back to the 70-300. My experience with the 70-200 was that it was very sharp and contrasty (that "L look") from 70 to about 135 or so, even wide open. But beyond that it quickly deteriorated, so that beyond 150 it was, in my opinion, unusable at any aperture. Both my 70-300s were considerably better from 135-200 (even wide open) than the 70-200. From 70-135, they were comparable IF you stopped down 1 or more stops. I'd say at f/8, these lenses are almost indistinguishable at the wide end, the 70-300 better at the long (does get soft beyond 250 or so). If I'd read reviews more carefully, I might have picked this up before buying the 70-200, although I think maybe my copy was worse than average. Now, the 70-200 f/4L *IS*
is by all accounts a fantastic lens. I think many people assume that the non-IS version is the same lens, just without IS, but this isn't the case. The IS version is a different optical construction, and sharper across the board, according to all reviews I've seen (also weather sealed, unlike the non-IS). This is not to say that the non-IS 70-200 is a dog - compared to the 70-300, it focuses faster and is built much better, the front element doesn't rotate on focus, it has full-time manual focus, and all zooming & focus movement is internal. So, I think the thing to consider is what you shoot. If you shoot moving subjects, neither of these lenses is great, but the 70-200 will help you more. Ring USM focus is better, and the constant aperture will be faster at most FLs (the 70-300 is at 4.5 by 85mm, and gets smaller rapidly). For walk-around shooting of static or slow-moving subjects, the IS in the 70-300 may help more. If you'll shoot on tripod, get the 70-200 if the wide end is more important, or the 70-300 if longer is more important or if you'll be stopping down anyway, as in most landscape shooting (don't forget to turn off the IS
For other reviews, see slrgear.com and photozone.de . Hope this helps.