I have a 7D and bought it with the 15-85mm this is my most used lens and I love it, at the wide end you can get barrel distortion and vignetting which can be improved by stepping down a tad but that is to be expected from a lens going down to that wideness, I think you will see an improvement over the sigma lens and certainly the IS will assist when the light drops off. Longer term I am looking to get the 24-70 L and the 70-200 F2.8IS but that is a long way off.
The 24-105 is a good bet but it is a lot of money and you do loose out at the wind end especially when you take the 7D 1.6 crop factor into consideration.
If you want to get a bit techy then have a look at these:-
sigma
Canon