The Tamron is $414 new from B&H with the rebate. And yes, unfortunately you would have to sell it when you upgrade to full frame. I recently sold mine for $360 on Amazon and got $318 after all the fees. I still feel it was worth it. Now that I have a full frame, I don't have a general purpose lens. If I want the 24-70 2.8, I have to pay around $1200 and it is heavy and huge. I really miss that lens. Here is a review that helped me make my decision and maybe it will help you.
http://www.the-digital-picture.com/Reviews/Tamron-17-50mm-f-2.8-XR-Di-II-Lens-Review.aspx I would also say that other brands that are not Canon, including Tamron, make some rubbish lenses but this particular lens is not rubbish. This was my first non-Canon brand lens and it beat out all the previous Canon lenses I have owned. Once you go full frame I do suggest sticking to Canon, though. For the crop camera, I was extremely happy with the Tamron.