I realize this topic has been beaten to death but if I write a thread maybe it'll help me resolve this issue by putting my thoughts on screen, plus I'm familiar with the technical aspect of each, but I'm more concerned with real-world application, results, etc.

I need some anwsers from people who have experience with BOTH and perferably own BOTH.
Is the f4 L IS as sharp as the f2.8 L IS at f/4?
At the same aperture is the Vignetting, CA, barrel effect, corner sharpness, and speeed to focus in low light equal at the same aperture?
In real life situations, if you have both, do you ever leave the 2.8 behind because of the weight?
Has anyone taken side by side shots (portraits) with the 2.8 L IS and 4 L IS and compared bokeh and subject isolation?

Outside and inside what conditions do you find yourself taking the f/4 off and putting on a f/2.8 OR wishing you had a f/2.8 IS?

I have Rebel XT and XTi bodies and have recently rented the 100-400 L and the 70-200 2.8 L IS but have had trouble finding a rental 70-200 4 L IS locally. I loved both the 100-400 and the 70-200 and found them not too bad carrying them around.

(the 100-400 I used during a Dragon-boat regatta over two days and the 70-200 I used for a wedding ceremony and reception in a dark theatre with theatre lighting. Both performed great for the application that I tasked them with)
Does anyone take the f2.8 L IS hiking/backpacking?

The type of shooting I di can be sampled at (www.vancityvegans.com)
I currently own a Sigma 18-200 3.5-5.6 and a Tamron 70-300 LD 4.5-5.6. Both focus slower and aren't as fast or sharp. The Sigma has serious Vignetting and softness.

I owned the f2.8 and loved it, but it was too heavy to use as much as I wanted to. I bought the f4 and compared them and found that the f4 was much sharper all around. I ended up selling the f2.8 and keeping the f4. I found that when looking at my old photos, I rarely used f2.8 and most often was shooting at f4 and above. I'm sorry, but I've deleted my test photos that would have helped you, but I used a distant building with lots of horizontal and vertical lines, colored signs, etc. and found a real diff in the color, sharpness, and overall pleasing "feel" of the f4 at all focal lengths. It was a no brainer for me to keep the f4...especially since it weighs half of the other lens and is a bit smaller overall, so fits better to my needs. I LOVE the IS and find it necessary for many of the shots I do of animals.

You can't go wrong with either lens, but the f4 will save you a few more $$s to buy another one!
Jan

I have recently seen statements claiming that the non IS 70-200 is indeed sharper than the IS as well. have you considered purchasing the 70-200 f/2.8L non IS? It should run you more of an f/4 IS price, but will give you the f/2.8 you may want. the non IS is also lighter than the IS version, though not at light as the f/4 IS. after looking at your site, I really feel you may want the f/2.8. f/4 is quite a bit slower, and if you ever shoot low light you will most certainly notice the difference (although the IS claims to be able to imitate a higher shutter speed). and you do have some beautiful images on the site.

After looking at Trex's website, I see he really needs the 2.8. Being a gal, I found the weight an issue....guys wouldn't. Also, I agree...had the non IS f4 which I felt was a very nice lens, but I do wildlife, so like the IS and found it works best for my photos. I'm planning to rent a 100-400 IS for my next trip to AK so am eager to see how that performs.
Jan

I have a 100-400 IS and after using it for over a year I'm looking to replace it.
On the other hand I have two 70-200 L IS 2.8 in case I drop one again :eek:

I found that in the rain the 100-400 waterlogged and fogged up, yet the 70-200 remained perfectly usable, because of the internal zoom on the 70-200 and the push/pull zoom on the 100-400.
Weight isn't much different, in fact when I get out the 100-400 I feel it's lighter than the 70-200 !
From Canon specs, 100-400 1,380g and 70-200 1,570 g so it's only 190 grams but I notice it.

I find it soft wide open at 400mm, fast to focus at any focal length, with vignetting at 400mm wide open but great stopped down to f/9.
I'm now looking for a faster lens for wildlife and motorsport.