I've run quite a few m/r queries against 2.4.2. The performance is still poor enough that'd I'd say it's only usable for occasional ad hoc work.

More seriously though running queries against a master node will regularly (maybe one in two queries) cause it to failover and elect a new primary. You'd think you should be able to run the queries against a slave, which you sort of can. Unless the result set is bigger than the maximum document size (16megs), in which case you're limited to the master as you can't write to a collection on a slave.

We're using Mongo fairly successfully but it has a lot of issues, particularly around administration tasks. Map reduce work gets done in Hadoop.