Our PagerDuty is integrated right now with Scout, Pingdom, and our own custom alerting system.
So far most complaints we've had while using the service for the first week were our own faults: for example, our monitoring was too sensitive, which was fixed by using the regexp filters, and by eliminating spurious errors from reporting on our side. One thing that PagerDuty did was that it basically forced us to fix these reporting issues so that we weren't woken up at 5AM unless it was a real emergency.
The SMS interface got a little confusing when we had two errors at once. For example, a frequent case is getting two pages at once, "Service X is DOWN" and "quora.com is DOWN". I think what I tried doing was:
1. Receive the first report (site).
2. Receive the second report (service).
3. ACK both reports using the second report's code.
4. Fix the service.
5. Attempt to resolve the second report, receive a "code already used" error.
Resolving things via SMS is a little bit clumsy (it's what I usually default to). A link to the PagerDuty login would be cool, but I don't know if it would fit in the 160 character limit.