I'd like to add that before wp3.6, even though the database cache is set to disable for logged in users, w3tc still manage to cache some of them. I'm not sure if it's a bug or feature. But after 3.6, caching for logged in users are completely disabled.
From where I stand, it looks like I'll have to implement custom database caching mechanisms outside of wordpress.
I really wish that you guys could fix the object cache feature, and make it work with memcached, because this API is surely one of the best features wordpress has to offer.