I've run into a few issues with the caching done with WP-FFPC - although I'm not sure if there are solutions...
But I have two themes, one of which is used automatically by mobile browsers. It seems that if a page is visited by a mobile browser first, it will cache that page and serve it to non-mobile browsers as well. Is there any way to disable caching on a per-theme basis?
Second, if I am also using nginx and try to use the memcached cache directly, when I access individual post pages as a logged-in user, it renders it as if I'm not logged in - my guess is having nginx using the cache bypasses any options that are set in WP-FFPC and serves up whatever page is cached regardless if I'm logged in. This could be very bad if I allowed it to cache pages for logged in users, as the users might not get the logged-in version of the page, and conversely non-logged-in users might get the logged-in version of the page. This makes it unsuitable for sites that have general users.