What to seek/avoid in a hosted server monitoring service?

I'm looking for hosted monitoring solution (CPU, memory, disk, loads, mysql, replication network etc..) for a group of servers on Amazon EC2 /Scalr (app, mysql, load balancer).

So far I found http://scoutapp.com , http://www.serverdensity.com, http://portal.monitis.com/

Do you know the pros and cons for these services. do you have experience with them? Any other similar services I should look at?

Thanks!

From serverfault Niro

• how about zabbix with http://www.mikoomi.com/ it all open source

  Niro : Thanks. I have zabbix right now. Dont like the UI
  From Rajat

• How about Circonus:

  https://circonus.com/

  It's made by the guys over at OmniTI. I've been using it for months and it's an excellent hosted monitoring and trending solution. Prices are very reasonable too. I highly recommend checking it out.

  Niro : Just checked it out. It looks more like a pingdom competitor to monitor web pages load time than a serious server bottleneck level monitoring tool

  vmfarms : Not at all, it goes well beyond that. It can use a Nagios-like agent on your systems to report data back to their interface of any shape and size. It's quite a powerful system.
  From vmfarms