+1. I've used Cloudkick (before Rackspace), Copperegg (not too bad either) and New Relic. Server Density is VERY reasonably priced, simple and gets the job of server monitoring done. The custom metrics are key. New Relic for example doesn't support custom metrics. A custom metric example: my Sidekiq queue sizes. When Sidekiq first came out, there was no way to alert/monitor this. It only took a few hours of Python.
What they are lacking in comparison to running your own NMS (e.g. NetXMS or Zabbix) is making custom views (the Dashboard feature in NR is pretty useless).
If you liked Cloudkick I think you should also check out https://mist.io. In fact we started mist.io when Cloudkick was about to close down. We like to think of it as mobile friendly Cloudkick with a twist.
