I've used many different payment processors both for my own ventures and for my day jobs.
Recommended: Google Payments
Tepid: Paypal, FirePay
Not Recommended: Authorize.NET, TotalTrans, 2Checkout.com
Paypal was the only one I'd done using Rails (the rest with PHP). It was a pain in the ass, but it was also 2005 and Rails was at version 0.13. There is probably a gem or plugin for it now (I implemented it by posting and parsing raw POST requests).