I been there recently trying different copies of the 24-70. Do remember that dof is quite shallow especially close range, 70mm, wide open. So shooting on a tripod would be preferable. That said I donīt think the examples are particular bad.

Dawei: Can be a tad challenging on a 20D without focusing screen.. I have to MF my Sigma 150mm as it hunts horrendously. At 150mm or 135mm it is easier to focus I suppose. When I tried MF on my 16-35mm I had a 10000:1 rate of keepers...

J. Andersen: Basically this is roughly what to expect from a Canon L zoom?

Can it be your 20D? Some of the comments made would make me wonder. I had 2 20D's at the same time and did some testing in the basement on various lenses such as the 28-70L and Tamron 28-75 F2.8. Two things came out of that testing, and since I had 2 20D's I was able to narrow it down a bit further. First thing was the 28-70 was sharper than the tamron on the long end while the tamron was shaper on the wide end. Also, the 28-70 takes sharpening better and was clearly the winner. The other thing (which is the reason I am posting) was the viewfinder was misaligned on one. So the AF Point I was choosing was not in the location shown in the viewfinder. I thought it was my problem but turned out to be equipment. Also during the testing, I found that the AF system/lens focusing was indeed affected by florescent lighting (in certains situations and with certain lenses) I could duplicate AF (one shot, tripod) issuses (back focus typically) with several L lenses. However, when using the Tamron in the same testing, there were no issues, so I speculated those new coatings they're puting on lenses designed for digital do make a difference. So, I agree with taking the 28-70 outside for the test and no rely upon one done under inside the store conditions.That's only from my own experience.

Jim: Thank you for the tip. I will probably rent a copy for a day and give it a good and proper testing.

I once owned the 28-70 and it didn`t do anything for me, I sold it. Maybe today and using live view to focus might have been different.

I noticed before, however today it really got to me (slow, eh?) that my AF seems to be a little out of order. Using 50mm f1.4 I tried to focus on a line and several times focus reajusted after confirmation. I am not 100% sure, but I believe I seen this on my 350D also.
This surely could have affected the test. Going to the shop today to do another test.

My 28-70 backfocuses about 0.5 cm (~1/4 inch) consistantly at close to the minimum focusing distance and 70mm. You might try to see if this is the problem you were encountering. Even at 5.6, if the lens backfocused, there are times when it wouldn't be quite enough DOF to maintain clear sharpness throughout.

My copy is really sharp otherwise. I've cranked out winner after winner with the damn bugger, at a more than fair price (US 650). I am far too lazy and cheap to send it in for calibration, but I suppose I aught to one of these days. When I get my 5D II...Right now on my 30D the focal length leaves me consistantly reaching for my 20-35L instead.

PetKal wrote:
My 28-70L is a good lens.
My 24-70L is a better lens.......a bit sharper for one.

One really has to shoot those two lenses side-to-side in order to be able to compare more reliably.

Obviously, I can not say how representative my 28-70L is of the whole population of that model. All I can say is that I bought the lens in a true LN condition and the focus calibration is right on the money.

+1

Here are three examples of things I shoot with the 28-70L. All around a dependable nice walk around lens. Obviously, 24mm is more useful than 28mm...........I miss those 4mm when I am using the lens.

Overall IQ and colours very good. Bokeh good, but not quite on the 24-70L level.
Three more samples.

PetKal: The shots look great. As opposed to you, I find myself needing extra focal length when shooting 16-35mm... I suppose one is never content with what one has got..hehe

I seriously need to stop reading reviews here. 28-70mm seems to be most popular lense.
The-digital-picture seem to agree on the 24-70 and 28-70 comparison.

Krosavcheg, photography forum folks often tend to over-compartmentalize the application of diverse gear types and models. As one does increasing amount of practical photography with various gear types, one learns that different equipment models are often of marginal impact on results. Yes, theoretically, the 24-70L might be a bit better all around lens than the 28-70L, yet in practice those differences are negligible when compared to our own shot visualizing and execution skill.
Furthermore, if all I have on me is 16-35L, I will apply myself to work best within the limitations and strengths of that particular lens.
Here is one more 28-70L shot which I like.

fwiw the PQ like PetKal is posting (even pixel peeping larger images) is what i expect & see with the 28-70L. if $$$ was no object i would upgrade to the 24-70L to get the extra 4mm on the wide end.