I bought a used Canon 20D last summer and the 18-55 lens it came with is really starting to get on my nerves. It feels really cheap compared to what I used to use with my old AE-1. Unfortunately, my old lenses don't work with the new body so I'm looking to rebuild my lens kit. I'm looking to buy a decent 18-55 or thereabout lens and I am really confused by the number of choices I have. I have three big questions when it comes to buying lenses: 1. Is it safe to buy used lenses? (Never even considered this in the days of film) 2. Is IS really worth the difference. 3. How do Sigma and Tamron compare to genuine Canon? Any advice would be most appreciated. Thanks.