I'll pipe in, too, as I usually do when these 17-50 discussions come up. I've got the Tamron, and it is a very sharp lens. However, it took me two copies and a calibration by Tamron to get there. Both lenses had big-time focusing issues. Even the first one, which was the praised Made in Japan model, did not work well. While the factory calibration made a huge difference, I wouldn't recommend anyone do that if they rely on the lens. I recall that it took about 7 weeks to get it back, and to boot, the aperture blades were stuck open! To their credit, they paid for overnight shipping both ways after I complained, so that mistake was quickly resolved.
Mine also requires a -0.3 to -0.7 EV setting to keep from overexposing. And I often have to dial in some additional negative flash compensation as well, because it generally overexposes with flash. Obviously these issues can be worked around, but I find that my Nikkors and my Tokina don't exhibit this problem.
For someone on a budget, I think it is a great lens. Additionally, the smaller size and lighter weight vs. the 17-55 can be a big plus. I would love to have a 17-55, but it's still hard to justify the 2-3x price difference, even if I felt like I could afford it. And I will add that I once rented a 17-55 and did some comparisons between it and the Tamron. While my tests weren't scientific, my overall impression was that sharpness was similar, with a slight advantage going to the Nikon. It was the contrast and color rendition, however, that really caught my attention. By themselves, the Tamron pictures looked just fine, but when they were compared to those from the 17-55, there was a richness missing from them. You can tell from my profile that that "richness" is not yet worth an extra $800 to me, but I think that eventually I am going to pick up a used 17-55. I . I think 3rd party lenses can be part of a really good kit, but I ultimately want the lens that I use 95%+ of the time to be a Nikon.