The 70-300IS is fine for its price. Served me well enough for the time I had it. My copy was reasonably sharp and IS worked. Here are some examples:
http://rkkwan.zenfolio.com/houstongrandprix
If you decide to upgrade, you should get at least the 70-200/4 IS. The non-IS version is older with different (i.e. inferior) optics, so it's not just the IS.
[Anyways, I've owned and used quite a few Canon telezooms over the years. First is a Gen I 75-300 which I actually gave away to someone on another forum. I didn't need another paper weight or door stop. After the 70-300IS, I have gotten the 70-200/4IS, and now using the 70-200/2.8 IS II.]