I tried the free developer version of RightScale a year or so ago. RS offers a slightly different service than CK. With RS, you can create scripts which run at startup and configure the instance. RS does automatic master/slave DB server failover, automatic instance creation and load balancing, etc... I'm not sure how their monitoring tools compare to CK. I wouldn't say they are direct competitors, and if they are, RS can do quite a bit more.
That said, I really like the CK interface and will give them a try for basic monitoring.
Yes, just over 1.5 years at ShareThis. ~150 nodes managed by RightScale for ~$4000/mo (Premium account plus an add-on account for developers). In this case, the "managing" includes rich metadata, server provisioning, configuration management, deployment scripts, monitoring, alerting, etc. You can basically think of it as [Chef/Puppet/CFEngine]+Nagios+Cacti+AWS console+misc other tools, all wrapped up in a pretty (and very functional) GUI.
CloudKick's pricing puts it at about half that of RightScale (at our scale - I'm not sure how RightScale is priced at the bottom end), but it looks like the features are monitoring/alerting focused - the management tools appear to be largely undeveloped.
CloudKick is certainly making good headway feature-wise, and I'm glad to see them spin up their revenue stream. I look forward to seeing what other features they add on in the future.
That said, I really like the CK interface and will give them a try for basic monitoring.