I picked up a Sigma 70-300mm for a friend of mine off of Ebay for $105 about 3 weeks ago. I sampled it for a few days, before giving it to him. It's a nice lens for the price. Definitely better than the dreaded 75-300mm Canon!
However, if you are expecting massive improvements in the sharpness dept. over the 18-55 kit lens, you may be disappointed. The 18-55 is decent for a low-end consumer lens, and the Sigma 70-300mm didn't seem (to me, at least) like much of an improvement.
If at all possible, I would try to spend an extra $75 and pick up a used copy of a 55-250mm. [My friend was on an extreme budget so we had to stay lower!] That really is the "best" of the lot for light, consumer grade zoom lenses. I think you'll like the extra width on the wide end as well of the 55-250.
P.S. If you do go with the Sigma, make sure it is the APO one....