we use mongoCloud(previously MMS) in production for half a year now.
tried both Modulus and Compose meanwhile – no good for our needs.. and alot less flexibility.

some info:
~ monitoring and backup agents can be set up on a separate instance.. no need to install them on all the hosts.
+ there is a nice “scoring” system that notifies you via email how secure your mongo deployments are
– I am messing with arbiters now… short tip – don’t use them for now, as they can not be promoted to secondaries. just 1pri and 2sec
– you can not “easily” control ports that mongoCloud will use when deploying new hosts (just breaks my own naming convention :/ )

also.. use ReplicaSets.. always.. even with 1 host – this will save you some time when/if you’ll decide to add a couple more hosts to a cluster

and as a separate question: I am wondering will Meteor-MognoDB-Client auto-discover new(added/modified) MongoDB hosts without changing MONGO_URL(i.e. adding mongohost4:27002)?