Well I've worked with pretty much every provider available in the UK, so here are some brief opinions on the bigger ones: PayPal: Awful, API changes regularly without warning (including changing parameter names for no apparent reason) SagePay: Truly awful, datacentre seems to catch fire regularly WorldPay: Awful, but not as bad as SagePay Barclays ePDQ: You have to fill out an Excel spreadsheet to get a developer acc…
I'm not sure what you're doing with paypal, but there haven't been any API changes that have affected us in the 7 years we've been using them. In fact I just looked at our paypal code, and it hasn't really changed at all since 2007.