sbesaw Loc: Boston

You can check out this comparison review of two of the lenses
http://www.ishootshows.com/2010/10/18/comparison-review-tamron-28-75mm-vs-sigma-24-70mm-f2-8/

Tamron compares very favorably. Despite the need for some to trash one lens over another there are Pro's who make 100% of their living shooting Nikon, Canon, Tamron, Sigma and Tokina glass. Each have some special glass like the 11-16 Tokina which is a highly touted lens. Mike Moats, highly acclaimed Macro Pro shooter, uses Tamron Macro Lens and they are good enough for his award winning Photo's.

In this comparison: http://www.16-9.net/lens_tests/35mm_e2.html
the reviewer felt the Tamron outperformed the Sigma and he includes shots and data. Worth a read even if you dismiss it.

Here's another Forum discussion on these lenses with reactions fairly well split: http://www.thephotoforum.com/forum/photography-equipment-products/250852-sigma-24-70mm-tamron-sp-28-75mm.html

Lastly great discussion with some hard data from the Pentax forum on these lenses: http://www.pentaxforums.com/forums/pentax-slr-lens-discussion/99689-sigma-24-70mm-sigma-28-70mm-tamron-28-75mm.html

alggomas Loc: Wales, United Kingdom.

With Sigma, they seem to have problems [as far as I know ],with Canon cameras. You can get a good one but is trial and error sometimes.
The Tamron is the better bet.
I would look at 2/3 reviews of the lenses and then make a decision.

daddybear Loc: Brunswick, NY

I have Canon, Sigma and Tamron in my bag,vest. Canon nifty 50 is great, other Canons are very nice, Tamron 17-50 2.8 is also great, Sigma 70-300 is a good lens. Sweet spot is about 270mm and then step down one stop. Use them all. I am about to purchase a prime lens in the 80-85mm range. Probably a Canon but Tamron is in the running.
Perkins Cove was taken w/Tamron, hand held from a footbridge over the harbor inlet.

Perkins Cove, Maine

alggomas Loc: Wales, United Kingdom.

I bought the Canon 85mm f1.8.
Lovely lens, but you have to use your feet as a zoom.
Good indoors.
Here in UK about £270 discount.

Spodeworld

I have the Tammy 28-75 f2.8 for my Nikon and it is very sharp.

daddybear Loc: Brunswick, NY

That is one aspect of the 50MM I am learning to like. Manual focus
and moving my feet has become enjoyable. I had a lung transplant about a year ago so walking is good, can't go too far but it is good.
$389.00 seems to be "street" price at the reputable shops like Adorama, B&H.

imagemeister Loc: mid east Florida

Canon 24-85 USM , about $200 used on e-Bay , ....great lens !