Personally I've never met a Tamron lense that I've liked. See this review:
http://photozone.de/8Reviews/lenses/tamron_1118_4556/index.htm
1mm or 2mm at the wide end = a lot less width on a 1.6x crop camera. You'd be surprised. Anyways, if I was going to give up some width for affordability I'd probably get the Tokina 12-24 f4 lense. Same price range as the Tamron, but a better lense in my opinion, plus it's f4 across the whole range, NOT f4.5-f5.6. That extra 1/3rd - 1 full stop makes a big difference in shutter speeds for lower light, handholding, etc.
http://photozone.de/8Reviews/lenses/tokina_1224_4/index.htm