I ran XO for a few years at my office and was one of their first SIP installs. We have had a few issues. I switched to Wave a few months ago and so far so good. We terminate to a Cisco CallManager Express system. I have a client that uses Verizon for SIP (nationwide) and there have been ups and downs (not necessarily related to the SIP delivery). My last outage on XO, they pointed the finger to AT&T's router one town over (the disadvantage of not being a tier 1 carrier). Our phones were down for a full day.