Agreed. Authorize.net is problematic. Not really because of Authorize.net, but because you have to tie it to merchant services--and every one I've used with Authorize.net has been uniformly awful. Everything is targeted to businesses with card reading equipment, rather than online processing--all of the folks at the merchant services companies are baffled by questions about online stuff (and Authorize.net refer you to the merchant services people for a wide array of queries). It's just a poorly matched set of tools.
API support for Authorize.net is excellent, though...that's the only reason I use it. Some weekend I'm going to convert to Google Checkout or something else.
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).