mrjob looks nice. If you have a Hadoop cluster. But for "medium sized big data problems" FileMap is a very viable alternative if you have people who know their way around a terminal. Especially if you'd like to have something set up pretty fast. For 500 GB of data setting up Hadoop (steep learning curve; name nodes, jobtrackers, thrift API:s, datanodes, zookeeper and whatnots) is a lot of heavy lifting. Not to mention administering it.
Sure you have Cloudera et al., but I'm still trying to figure out if they really make things easier or worse when it two weeks later comes to figuring out why something is broken, or how to start/install additional Hadoop components.