We're using the beta. They charge US$0.25 a transaction, and US$1.00 for a failed transaction (e.g. insufficient funds, frozen account, etc). No %age of transaction is taken off the top (yet). API works as well, though their client libraries aren't quite up to date with it yet, and you have to setup a webhook because ACH processing takes several days.

What would be a game changer for Stripe is to have ACH processing in the realm of 1-2 business days instead of the 5-6 days it currently takes. This would probably require some fancy exchange of money between accounts behind the scenes, but if they can pull it off it may be worth paying for.

Also, though it's US$0.25 a tx, it's actually on the order of US$0.15-0.20 at some other places. But for us our volumes are low, and their API makes it worth it.