Hourly incremental backups

[parasquid]

I've tried searching the forums but I couldn't find any similar use
case (or maybe I'm starting to go blind ...)

I'm using MongoLab as a hosted mongodb service and while it's nice
that I don't have to maintain the server uptime myself, querying their
servers for some adhoc internal reports take a tad bit longer than I'd
like. So I thought of maybe setting up a local mongodb server (no
replication, no redundancy) and have it sync with the MongoLab
database hourly.

Because all of my records have a timestamp, I was thinking of using
mongodump with a query and dump the last hour's worth of data using a
cronjob. Questions:
* has anyone tried this before? What were the results?
* is there a better way? I saw documentation about oplogs but I have
no idea if I can get the oplog for just the past hour and replay those
into my local db

Thanks a lot! :)