I am on a budget and I want a fast lens, and I found this

I am using a Canon EOS Rebel T2i, and I only have one lens, the 50mm f/1.8 II, its a great sharp lens which suits my need, but a few days ago I was asked to photograph an event, I dont know how will I manage with only this lens, a fast zoom is a must, so I checked the options, most fast lenses from canon are over$1000 which I still cant justify, so I checked the alternatives.

the 17-50mm f/2.8 XR Di II LD lens from Tamron is getting great reviews and its half the price  of other lenses ($649) like the canon like the EF 24-70mm f/2.8L and the Canon EF-S 17-55mm f/2.8 IS USM