For what it's worth, I spent some time working with Worldpay in the UK for some charity work I did - again, this was working with an existing infrastructure, so I had no choice of payment processor. I don't know how closely related Worldpay in the UK is to Worldpay here, but I found them pretty easy to work with - their stuff was simple and well-documented, and they were developer-friendly in terms of test resources and so on.
I wrote my own stuff to interface with them - partly from scratch, and partly translating an existing Perl setup to Python, on Linux of course
so I can't really comment on shopping carts or ASP stuff...