Payment Processing Software/Backend
 

I'm researching processors for a project I'm working on. Has anyone dealt with any of these products, and if so, good or bad comments.

QuickBooks
Xero
=Exact
FreeAgent
QuickFile
Sage 50
ClearBooks
KashFlow
Zoho

I'm looking for the easiest interface yet very secure, slight learning curve and something that won't eat cash for breakfast.