Hi Florent,
My impression is that the MongoDB and MySQL adapters get the most use.
The MySQL adapter had a bug in it until recently that caused errors if
there was no request in a 24 hour period, but that is fixed. I
personally use the Tyrant adapter, which is probably the least
popular. The Riak adapter is reliable but it is labeled "experimental"
because the find function is broken. I have been waiting in vain for a
knight in shining armor to fix it, but alas none has yet arisen out of
the ranks.
Evan