We haven't had a single problem with Mongo or API interfaces. This replaced MySQL + Memcache and was a MASSIVE difference in system load and performance.
Your mileage may vary, but it's very, very rare that I can say I haven't had a single real problem with a third party package (especially one as complicated as a database-type system), more so with early adoption.
(I'm still using Memcache for a few legacy pieces, but in general things I was having Beaker stuff in Memcache such as sessions and Pylons template caches are going into the Mongo Beaker plugin I wrote - http://bitbucket.org/bwmcadams/mongodb_beaker/ )
Anyway I think I'll try again in the future.
I'll build the production version when I have some free time and then I could tell you. (I was working with v1.3.4)