The Canon 17-55 F/2.8 IS is a very nice lens, if you can afford it, I do suggest it. But at twice the price of the Tamron here, I couldn't afford it at the time, and can't justify buying one now, even if I can afford it.
But it depends upon how often you take photo's in low light. The IS will definately help compensate for your movements, but it will do nothing for the movement of the subject of your photo. I just got back from a month in Paris, and definately would have appreciated the IS in the dimly lit Museum's that your not allowed to use a flash in. But I was taking pictures of stationary objects. If they were moving, it wouldn't have helped because of motion blur. So it just depends upon what your taking pictures of, and where.