We have been approached by a good dozen merchant services providers, but have stuck with the services offered through Sam's Club. Most providers offer short-term savings, but the stability and reasonable cost of the Sam's Club service has kept us from jumping around.
That said, I pretty much hate Sam's Club (personal reasons, nothing to do with their merchant services) and my partner and I have a positive impression of several other services that have approached us. At another time of year we would be tempted to switch.