When I started my business, my level of web programming expertise was "I can create a page in HTML in Notepad." Dealing with Paypal callbacks causing key generation, accounting, etc was way the heck out of my capabilities. Enter e-junkie. As I gradually improved in my web programming abilities, it always made more sense to extend e-junkie than it did to rip out and replace, particularly since they wrap both Checkout and Paypal in a consistent interface.

It works, costs $5 a month, is reliable like you wouldn't believe, and their lead developer literally sends me Christmas cards. Why would I ever change?

It is likely that my next product will not use e-junkie, though -- subscriptions, blech.