I've been using it since January. I had some snags with firmware using the broadband adapter (generates it's own dialtone for the security system and it dials directly into next alarm) and with some of the contact id communication formats.
You will get a communication failure alert sent out but I think it is about 2 hours after it fails to communicate. I don't think they poll the broadband adapter that much.
Feel free to post or email me if you have any nextalarm questions
I too have been using NextAlarm. I have a Caddx panel. I haven't had any problems at all. I was originally using Vonage to connect; however, I've since purchased their ABN adapter. I wasn't having any issues -- just wanted a dedicated line for the alarm.