I use New Relic and I'm not sure I would recommend it. Sure it was easy to install on all the server but now I keep getting alerts every months (on like the 2nd day) that I ran out of some "quota" of 100GB. I thought that's impossible as I almost don't use it but apparently they (used to) send by default list of running processes every second and that gunks things up.

Also I setup alerts - for >50% CPU or >90% disk full. I do get an alert but it doesn't say which volume or how full is it - what was the actual value that triggered sending the alert. WTF.