Good question. Originally I did do it all myself, I installed it and I had replication and all that set up, but it fell over and until you have a bunch of experience that means you're a noob who has to dick around in Google and IRC looking for answers for every simple problem, while part or all of your platform is broken.

Our servers are already in multiple datacenters which means we can either maintain our own multi-dc MongoDB cluster (not really an option w/ me at the wheel) or we have to eat some latency anyway.

In my opinion it's cheaper + easier to let MongoHQ handle it all, and latency .. as long as it's measured in milliseconds it's acceptable compared to doing it all myself.