Card Service International is one of the most widely known credit card processing companies out there. Upon starting up a small clothing shop in my home town, I set up my merchant account with CSI. Though it was far from smooth sailing, I had an overall positive experience with the company. It was somewhat difficult to discern who to call in order to resolve various problems -- my account rep, the equipment company, or a general service number -- but once I found the right people, my questions were answered quickly and honestly. And even though my store closed before our merchant agreement was up, they gave me a discount on paying off my contract, which I really appreciated.
Some other reviews of Card Service International can be found here and here.
Based on my own experience as well as the other reviews I've seen, I have to give Card Service Intl