Waiting for this lens to come out since August, I bought it last weekend without finding anything on the web but a few prices. This week I discovered fredmiranda.com again and... the first reviews!
All the reviews here tell enough already, especially the extensive report by spoli58, so here just something general to think about: its sound. I bought it without any information but the Canon specs and a price. The shop was noisy when I tried itout. At home I was disappointed first about it's AF and even IS sound (I can't hear IS on my EF-S 17-85mm). But if you know that and accept it, it's a great lens for anyone who wants a telezoom with IS. I have just an old EF 70-210mm f/4 from 1988 to compare with, but AF seems fast enough and accurate. I agree to other reviewers that with less light, at 300 mm, AF gets lost sometimes.
I haven't been testing its image quality much yet, except for IS were I was curious about (the main reason for wanting this lens), and I must say that it's great to have 3rd generation IS on a telezoom.
This is a consumer lens and therefor "less" perfect. I recommend this lens to any serious amateur.