Disclaimer: I work for MongoLab.
Using a database as a service like MongoLab truly lets you focus on application development instead of database operations. There's also a whole plethora of tools and features that MongoLab provides to make you more productive with MongoDB: web tools, monitoring & alerting, backups, and more (more info on our website).
MongoLab manages over 200,000 databases in 19 datacenters, which means we're constantly monitoring our fleet of databases and even auditing our own monitoring system. Most of the time our support team is paged about an issue before the end user even notices it. Alternatively if it's 2 AM in the morning and your production database performance is suffering (unindexed query being run by a remote developer perhaps :), users can email our 24x7 support hotline and we'll respond within minutes. Knowing that our systems and support team are always there provides a peace of mind that our users find incredibly valuable.
We offer free 500MB Sandbox databases that don't require a credit card and don't have an expiration date. I encourage you to check out our service and you can easily compare it to self-hosting a MongoDB instance. Happy hacking!
-Chris@MongoLab