PagerDuty is a fantastic service. The flexibility in scheduling on-call rotations (they added a custom schedule for my team at my last company), the support for integrating with lots of different monitoring platforms and the generally fantastic support make it well worth the monthly cost.
At Loggly, we integrated our alerting app with PagerDuty, and open-sourced the whole thing if anyone is interested:<p><a href="https://github.com/loggly/alertbirds-community-edition" rel="nofollow">https://github.com/loggly/alertbirds-community-edition</a>
Datadog hearts PagerDuty. We use it every day. And sometimes nights too...<p>And here's its awesome API in action in Datadog -<a href="http://www.datadoghq.com/2012/07/pagerduty/" rel="nofollow">http://www.datadoghq.com/2012/07/pagerduty/</a>
Is PagerDuty worth it for $18 per month? For a 5 person team its $1080 for an entire year to do something that a small script + Google Calendar could take care of. Maybe I'm missing something? I'd love to hear a story about how a feature of PagerDuty saved somebody's butt.