I've used Velocity and X-Trader for a couple years. Over the past year (if my memory serves me correctly), I recall 1 time where things were locked up. But it was no sweat because they have redundant servers so I simply had to shut X-Trader down, and relogin under the CME-B server. Overall, it is rock solid.
Cust.service is good too - haven't had a need to call them really...
I do like that they provide X-Trader free of charge as an option (but you have to pay more in commissions). I pay $5.75/roundtrip on ER2. If I want to pay $500/month for X-Trader, my commissions would drop to $3.75. I'd have to average at least 12 contracts a day for this to make sense...
Try 'em out! Before Velocity I was with Man Financial - their PATS system (or MTrade Pro as they call it) sucks in comparison.