Even though there is no doubt that the canon lenses are superior to most (all?) other lenses, there are many of us that cannot afford these. I am a low-budget hobby photographer who bought my 400D camera just before a trip to Uganda (2008). Since then, I have bought a Tamron 17-50 2.8 to replace the kit lens, and a Tamron 11-18 4,5-5.6. I am now looking for a telezoom to replace the canon 70-300 4.5 - 5.6 (non IS) which is too slow, and also show substantial chromatic aberration. My dream is of course the 70-200 IS 2.8, which is far out of my financial league. The 70-200 2.8 without IS could be a nice substitute (I have fallen in love with fast lenses), as is the 70-200 IS 4. However, there are two alternative lenses, Tamron and Sigma, both 70-200 2.8. I would like to know if anyone has got experiences with these lenses. Both Tamron and Sigma's versions are relatively new, and not reveiwed on this site. How much quality is lost by "stepping down" from Canon to Tamron/Sigma?