sigma has it's own 24~70/2.8 that costs about $400~450. it's a decent lens, and a good price for what you get.
tamron also makes a 28~75/2.8, and costs around $380 (350 after rebate). it's sharper than the sigma, almost as sharp as the L lens. but the build quality is not as good as sigma/canon. it's the lightest of them all, but i dont think the build is bad at all. optics is superior and i think it's the best mid-range zoom that's not the canon L.
i think those two are the 'fastest' you could find for a zoom with 50mm focal range. i almost bought the tammy, but i just felt it wasnt wide enough, and if i were to get the sigma, i started thinking i could pay twice the money and not only get exellence but peace of mind as well. crazy how a lens can give you a boost of confidence when going out to shoot.
[edit] i just thought of it now, but there are wide angle options as well. tamron 17~50/2.8 and sigma 18~50/2.8. they're just like their longer focal range counterparts. tammy wins in optics, sigma wins in build. there's no L lens version, but there is somethin that comes really close, and it's the 17~55/2.8 IS. it might as well be an L lens, but i guess the folks at canon didnt think so. tokina also has a 16~50/2.8 coming out that people are having high expectations for. i cant say much about it, but if it lives up to what people are buzzin about, that might be the perfect fit for a semi wide-angle + mid-range zoom lens (on a cropped camera of course)























