I work at a camera store and Tamron's always been my favorite brand. Build-wise their lenses are usually below main brand, but any lens in their SP series is guaranteed as sharp as other brand equivalents or your money back.
I was a little averse to getting this lens, as I work with a Canon 70-200 2.8 IS and other fast lenses for music and stage work, but I thought, hey. I get a nice store discount and I don't have any really FUN lenses. So I picked one up.
It stays on my camera now. If I'm going to shoot pictures for pleasure I leave my bag at home and carry along a little holster bag with this lens, my 430EX, and cleaning cloth/batteries. That plus my TrekPod and remote lets me do just about anything I want in a lightweight and convenient package. What's more, this lens focuses extremely quickly, and quicker than expected at f/6.3. The mag ratio is nice, and lets me get relatively close when I don't want to lug around my 180 macro.
Don't expect this lens to give you pro quality results, but once you buy this you might just find yourself bringing your camera to a whole lot more places.