We moved to TokuMX (http://www.tokutek.com/products/tokumx-for-mongodb/) recently and saw a 2x improvement in response times. They've swapped out mongo's backend with their fractal tree storage engine which is MVCC. Definitely worth checking out.
This is something I've considered pushing for at my company. Any pitfalls you encountered? Did it reduce db size on disk as much as their marketing claims?
My tests with it showed a drop in disk size of about 20-30%.
Unfortunately the read/write improvements weren't high enough to justify the massive engineering feat it would take to replace MongoDB with TokuMX, mostly due to the lack of commercial support for Toku.
Our disk size shrank to 10% of stock mongodb size.
The migration is easy now that they've released a tool to replicate from a stock mongo to TokuMX. They also do have commercial support that we've paid for and their team has been incredibly helpful and responsive.
