I've used it for caching, session and transient objects connected to Rails without issue for the last two years.
Additionally, I've been looking at it recently for a simple database to perform fast matches on sorted data (E.g. get me the lowest number in this set).
Why would you need to downgrade if you didn't have issues? ;) ...kinda joking, kinda honest curiosity.
And one more: - Storing diffs like those used for Google Docs (literally every change) for a real-time communication system.
The last one works by the client not getting a response and continuing to queue up its diff and send it once the server is back up. Manual conflict resolution is likely necessary after a drop in availability, but consistency is easy to figure out with monotonically increasing IDs.