I'm in the market for math postdocs and was just offered some positions. Essentially my situation is as follows. I have two offers from AMS Group I universities. One has higher "ranking" and is in the top 15, and the other in the 30s. However, the lower ranked university has a great group and there is a greater potential for collaboration.
I'm concerned that if I go to a lower ranked place, faculty positions at a good institution will not be easy to find. How important is it to go to a famous university for you postdoc? I'd also like to hear about the experiences of mathematicians who didn't go to a top 10 university for the their postdoc, but worked their way up.