We've been using TokuMX for a while and I highly recommend others take a serious look there. It already contains the benefits cited for MongoDB 3 and more. Toku also has a release candidate for the new Mongo storage engine API named TokuMXse, though it's interesting to see that TokuMX proper still outperforms it: http://www.tokutek.com/2015/01/first-tokumxse-performance-nu...
Problem is where it is branched. I don't think they will be able to keep up well. Plus, the new MongoDB storage engine (wire tiger) takes care of its biggest issue which Toku addresses.
Plus, I think you'll be able to just buy Toku's storage engine if you want it when they make it pluggable with the new MonogDB storage engine API.
