I'm not at all sure about the flat foot thing, but depending on how much you climb (and where), you'll be right at home in the FiveFingers...
I've cruised around bouldering areas in climbing shoes for ages. If you can handle walking around in climbing shoes, FFs will be second nature.
I personally don't use them for running, but only because of the sweat issue: because of the shape of my feet, I can't wear them with socks. My feet sweat a lot, so they need socks when I run. I've been using Merrell Trail Gloves with success. But I still like a little more padding when I run.
Quote:
Originally Posted by
Ambulance Chaser 
Bump. I am thinking of using my REI 20% discount and member dividend on a pair of FiveFingers. I don't run (my sport of choice is rock climbing), but may start if these are good.

Any other reviews? I have flat feet and a second toe that is slightly longer than my big toe -- will the FFs fit well?