I use Paypal's recurring billing API directly. It's not an ideal solution, but being based in the UK without a merchant account it's the only one I have available. I use ActiveMerchant and some extensions to talk to the Paypal API but I've had to write the logic to handle recurring billing myself. I'm trying to extract that logic into a gem and make it more robust. I'd like to eventually release it and possibly save some headaches amoung small developers who just want to get up and running quickly.