I don't have any production experience with it, but I did load the entire Bitcoin Testnet blockchain into it without issue. In Redis the same dataset required ~6 gigs of RAM.
Yeah I'd be interested in hearing that as well - I think most will take a slight hit in speed for something that's been successfully used in production with several well-known companies.
Does it manage its own cache like InnoDB? Otherwise you're gonna have a bad time.
Redis tried this (see http://redis.io/topics/virtual-memory) but it died a quick and well-deserved death (the code isn't even in it anymore as of 2.6).