Sigma 24-70mm f/2.8 EX DG Macro for Canon AF

Sigma 24-70mm f2.8
I would personally recommend this lens to almost anyone, with one exception being someone that shoots in very quiet environments as the focusing motor is quite loud. It is mildly annoying, but the shutter sound on the Canon 20d is quite loud as well and anything that is able to be shot with that sound is well able to be shot with this lens.

Pros:
f2.8
Very sturdy build, certainly lives up to the 'professional' EX line's name.
Very easy to switch to manual focus.
Colors are excellent, at least in the specific lens that I have.
Comes with hood and case, which even $2k Canon 24-70L does not provide without additional purchases.

Cons:
Loud focus
Sometimes searches for focus on plain objects for quite some time (up to 2 seconds sometimes)
Heavy, however it is a sacrifice you have to make for such a high quality construction.
82mm diameter filter size is more costly than the 77mm size, however you can purchase adapters, that is if you are not using a full format camera, to continue using your 77mm filters if you already have them.

Notes:
The constant 2.8 aperture(opening/amount of light that is let in, hence faster and better exposures) is extremely helpful for anyone that is planning on doing any sort of indoor shooting. At 2.8 it is slightly 'soft' (blurry) near the 70mm long end of the focal length, however I have not found it to be harmful to the photos and can be countered easily by stopping down to 4 or 5.6 aperture.
An interesting note is that the lens does not contract for the wide angle 24mm, it slides out farter, to about one and a half inches longer than it is when it is packed away.