Stripe charges 2.9% + $0.30 per transaction, the others mentioned have a monthly fee and then lower transaction fees. So Stripe isn't remarkably cheap.
Depends. Developing intregrations has a cost, and until you're processing enough to offset the difference, using something other than Stripe is bad business.
It is not cheap. It is however, remarkably well designed and easy to integrate with, provides excellent support via their freenode channel and allows for most of the outside of normal range issues to be easily handled on their dashboard.
I integrated with it for one of our sites and now have to integrate with cybersource for another and the difference in documentation alone is night and day.