The Tamron is a very good lens. In fact, it is very competitive even without the price difference, with the top grade manufacturers. Read the comparision
here. The focusing accuracy was even better than the Canon USM lens:
"outstanding AF accuracy, better than Canon's lenses"
and
"Within the row of the lenses around 100 mm the Tamron 90 clearly topped all its competitors, remarkable further the security of the AF. Though the Canon in this comparison performed better than in the "fern test", it does not reach the Tamron."
The Tamron 90 can be gotten at less than $550 the last time I checked with John 3:16 about 3 weeks ago. For that price, it is unbeatable.