How do you think the new Tamron 70-200 2.8 zoom might compare to the venerable Canon Mk II? I have a T1i and am currently using the Canon 135 f2 for indoor sports.
We won't know until it's here.
From past history we might infere, well, nothing conclusive.
Tamron's made some good lenses and then updates to them have made them optically better or worse.
You can bet Canon's build quality is superior, that's the majority of what you're paying for with their L series lenses. But if you don't need that ruggedness then the other mfrs offer functional and affordable options.
I'm hoping that Tamron's new VC version of the 70-200mm f/2.8 is at least as good as their current non-VC version which I have in F-mount. This lens is very pleasing for IQ, very sharp at both ends, a touch softer in the middle wide open but improves nicely when stopped down a bit. It's certainly much better optically than Canon's 70-200/2.8 L IS v1, not quite as sharp as the v2 but Tamron's bokeh is nicer. The Tamron doesn't AF quite as precisely as I'm used to with Canon Ls, and even MF, altho smooth, is too touchy with the short ring movement distance. Still, it's showing itself to be a very good lens even on the hi-res D800e. If they can maintain this level of optical performance while adding VC and an ultrasonic AF drive while keeping the price under 70% of the Canon or Nikon equivalents then it would be a no-brainer choice for non-pro use.
Just look at what they managed with their new 24-70/2.8 VC. Optically not quite as good as the v2 Canon but for the price, PLUS VC, I'll take the Tamron, thank-you. I think they even managed some weather sealing on it.
OTOH, their 17-50/2.8 crop lens without VC is a little better balanced lens than the later one with VC, the latter showing more de-centering issues altho better center sharpness at most settings.
Should be available in a few more months, then we'll know.