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.
Perhaps once my commercial agreement with 10gen expires in a couple of months I'll take another look.
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.