I just set up a site with probilling. They're easy to use and cheap. I will probably use them as my no.1 and acpay as no.2.
Probilling are working very hard and fast to get their system up to where it needs to be (i.e. customized signup pages etc.) but still lag a bit on the affiliate program side. They say their affiliate program will be upgraded in about 2 weeks. Judging by their progress so far, I believe them.
They have an open forum:
http://www.probilling.com/cgi-bin/ultimatebb.cgi