[elementai]

Evaluated it and a couple of others. Chose Bosun, never looked back, it's probably the only system with flexible and concise DSL for evaluating alerts (somewhat similar to R in spirit).

Riemann rocks, just not as monitoring system.

Bosun link: http://bosun.org/

[mVChr]

We evaluated both Bosun and Riemann for our use case and chose Riemann. Use case is over 10,000 servers with over 2 million metrics incoming every XX seconds. Riemann simply performs better, probably because it uses streams instead of a poller. The other part of our use case is to use it primarily for alerting, and it's fantastically responsive and robust for that, including simple integration with other systems (email, Slack, nagios, PagerDuty, etc). Bosun seems like a good tool for other uses however.