I'm in the middle of finalizing which lens to buy for my Canon 30D. I have choices of Sigma 24-70 f2.8 & Tamron 28-75 f2.8. I'm asking for your opinions on Sigma lenses in particular because recently came to hear a lot of complaints about the brand. Pls share your experiences with your Sigma lenses. Any comments on Tamron also welcome. Thanks a zillion!