ECHO is one of the major gateways. Our company has been using them since early 1996 (over 6 years now).
My personal experience with them is coding to connect our systems to their network and that was pretty straight forward. I don't have many complaints (if any) about them from staff that deals directly with them. So overall, they appear to be a good company to work with. Since we have a long time history with them, we most likely get better rates and terms than are available to new "unknown" customers, so I can't comment on that end of their offerings...
