I would really love to use them to monitor all of our servers but their current plans don't work for us. We use lots of tiny instances and virtual machines. This means for us the cost of Cloudkick for each VM would be around the actual monthly cost of that VM.
If you have are in a different position to myself and are using larger instances then I would recommend you give them a try!
That costs us $20/month, except we're doing it on a server that was already required for another service and had plenty of disk and CPU to spare, so ... really it costs us nothing, and gives us most of CloudKick's features, in our specific case.
Given their pricing, I think I'd rather take that money each month and pay someone to contribute to Nagios instead. :-/
But if we weren't on a tight budget (we're bootstrapped) I would have gone with something like CloudKick no doubt.
The server for Nagios might be cheap, but installing, refining and updating our monitors does take time that could be used on developing our product instead.
$99 per month for 6 servers is really cool.
What would you use instead?