I've logged a few thousand miles with a Selle An-Atomica, but only a few hundred miles with a Brooks.

FWIW, I put the Selle on my bike just a few days before the 2009 RSVP (Ride from Seattle to Vancouver and Party, ~190 miles in 2 days). I never experienced any break-in period, it was comfy from the beginning. The tension of the leather "hammock" is adjustable via an easily accessible hex screw. Other than an occasional adjustment, mine hasn't required any other maintenance.