So I'm on a business trip in San Francisco and there's a camera shop just up the street from my hotel. I stopped in and they're trying to sell me on this lens -
http://www.bhphotovideo.com/c/produc..._4_5_6_IS.html
They said they can give it to me for $599. Is it a good lens and, if so, is that a good deal? BH Photo has it for $649, but there are some on Ebay for as low as $300 (I don't know that I trust ebay for buying lenses though). They told me the ultrasonic AF motor means super-fast focusing which would be good for what I want to do. Also, it appears to have a metal lens mount which you guys have mentioned is better.