It's been great -- setup was a bit of work (we're on Ubuntu, so had to build from source), but once up and running it's allowed us to do lots of ad-hoc analysis that would have been too hard otherwise.
I've been meaning to write a MoSQL equivalent for our Impala data, but at the moment we're doing a more traditional ETL.
We've been pretty happy so far. There have been a few rough edges getting it up and keeping it running, but we've been very impressed with the performance so far.
I've passed your comment on to Colin, who wrote the Ruby client -- I'm sure he'll appreciate it!
I've been meaning to write a MoSQL equivalent for our Impala data, but at the moment we're doing a more traditional ETL.