My company uses EAutomate. I know they do tons of tickets per day (supporting in excess of 4500 locations). On my end I use RemoteTech which is the enduser version that allows me to handle what I need in order to close my tickets and such. It is from ECi Software Solutions, Inc.
It works but could work better. The last two upgrades were steaming piles for a while. The last one wasn't as bad but is still annoying (as in you are typing in your notes and it "helpfully" saves your progress in the middle of it so the second half you typed into nothingness and have to redo). The one before it broke most of the system for over a month. We host it all on our own servers and have periodic downtime, but I don't know how much is ISP and how much is system issues.