Sigma, Tamron, Canon

I have a Canon XT and I am wondering if I should stick with a Canon lens or if the lenses made by Sigma or Tamron are as good. How do Sigma and Tamron stack up?

Well all three companies can produce excellent lenses along with some real crap lenses..You should do your research on the lenses your interested in.

What range are you looking for, I'm sur ethe people here are more than willing to point you in the right direction.

Every time I look up a lens online and then read the reviews, the Canon's always have the best things said about them. One thing I am noticing with consistency is, for instance, with Sigma lenses. I am sure they are very good and I see so many people have them and say good tihngs about them but there always seem to be a BUT in there. Some will mention a soft focus, or a problem with auto-focus and some reviews I read mention more noise on the Sigma or slightly slow to focus. That is not to say that they do not perform well but for the most part I don't read those things when the Canon lenses are reviewed.
The other side of it is that I read reviews from people that absolutely love their Sigma and Tamron lenses and think they are tack sharp, recommending them highly.
Personally I don't know what I am going to do when it comes time for me to invest in lenses, which I am anticipating will be shortly after the purchase of my XTi. Until I am actually using a lens I don't know that I will really know because so much of this is subjective unless you just outright have a defective unit.

I own one Canon, four Sigmas and have a Tokina on order. I do photography as a hobby, not for money. For my needs, the Sigmas provide very good value for money. I could afford to buy all 'L' glass, I merely choose not to.

Depends on the lens. Overall, Canon's lens lineup is the best, however Sigma, Tamron and Tokina all have some real excellent lenses.

Off the top of my head, Sigma makes the 100-300 f/4 EX and the 120-300 f/2.8 EX, both of which have no equivalent in the Canon system, and they are among the sharpest zoom lenses available for SLR cameras. Absolutely outstanding lenses. The 24-70 f/2.8, 30 f/1.4 and the 10-20 DC are also excellent.

Tamron's 28-75, new 17-50, and their 90mm Macro are all very well reviewed. Sharp, high quality glass that doesn't meet Canon's L's in build, but are optically excellent.

Tokina has a very small lens lineup, but the build of their lenses are top notch (L-quality). Several of their lenses are absolutely outstanding. The Tokina 12-24 (which I own) thier 105mm macro and the 28-75 f/2.8 are all excellent. Tokina's consumer grade lenses (like the 80-400) are usually a step above other's consumer grade-glass.