I love my canon, it's a fantastic lens. If you can get a used one from a good dealer (with some kind of return or warranty) go for it. It's my main lens. I like it so much that I will hold on to my 40D once I go full frame, so I can continue using it.
I would suggest to put a filter on the front though. I always have one on there (usually polarizer, otherwise skylight) and I read in places that people ended up with dust inside the lens, I guess there are small holes somewhere that allow air in and out when you zoom. Supposedly a filter helps prevent that. I did not further investigate, since I always have filters on my walk around lenses, but something to consider - and watch out for when buying used.
When I go full frame I will most likely get the Tamron 24-70 instead of the non-IS canon (that costs 1000 more) since it gets great reviews and I often benefit from the IS in the Canon.
That being said, if you can get a better deal on the Tamron, you'll be just as happy I'd think.