I just recently bought a couple UPSes from RefurbUPS.com. I bought a pair of SmartUPSes -- a 620 and a 900. The 620 works great, no problems whatsoever. But the 900 is being a royal pain in the ass.

On the USB-connected systems, I get a lot of 'vetoed power requests' and such and it is not uncommon to get the 'lost communication' messages either. The USB is just tricky. Go with serial if you can ...