I looked into this and really like the idea of the service, but found there was one shortcoming that I couldn't get around. Perhaps I missed something, but I needed a much shorter escalation window than 15 minutes. If my site is down, I cannot wait for 15 minutes for the next person to be notified if the original person doesn't respond. It needs to be within 3-5 minutes, maximum. Perhaps this was configurable and I just missed it.
I really like this service and should the escalation change I would sign up in a heartbeat, it's exactly what I was looking for.
We left a comment on UserVoice, but I thought it couldn't hurt discuss here.