Have been reading a lot about this lens. Currently have the 17-40 f4 L, not useful for weddings. I was going to trade it in and get the 17-55 for my 20D. Should I put down the extra money and go with the 24-70 L?
24-70L is my most used lens to cover weddings alongside my 70-200L/2.8 IS.
If you have any plans for a future FF or 1.3X body, I'd hang onto the 17-40L and invest in the 24-70L/2.8. If not, than it makes sense to go for the 17-55.
In your current position (with the 20D), the 17-55 will likely be better than the 24-70L. Firstly, it's wider - good for those group shots. Secondly, it has IS in addition to being f/2.8 - therefore it has a better low light advantage versus the 24-70 (assuming there's no fast movement).
IMHO, the 17-55 is the ultimate wedding lens, thus I'd be keeping a 1.6x body in my system even I move to FF. Unless Canon decided to give us a 24-70 IS, but that would be an uber expensive lens I guess.
I wouldn't even think about shooting a wedding without the 17-55 f/2.8 IS. It is bright, has the perfect range for many wedding shots, and sports high IQ with IS. It's a beautiful lens. I tend to shoot weddings with two 30D bodies. One has a 17-55 mounted, and the other has a 70-200 f/2.8 IS mounted. An EF-S 10-22, and an EF 50 f/1.4 (among other things) are carried in pockets in the fishing vest that I wear under my suit jacket.
It's not the only lens that will get the job done, but it is a great lens for the job.
