I feel weird replying to all of these comments but here goes:

We were worried about Cogent at the beginning so we only used them for delivering some static files and for non-critical outgoing traffic. It was initially an experiment - to save on our in-datacenter bandwidth and to compare with Amazon (a very inexpensive experiment) and we've been very happy with it.

I think that we are doing about 20% of our traffic (+ offsite backups) over Cogent, 40% on our datacenter's fancy-pants bandwidth, and 40% on Amazon.