[NiteWave]

xcache has opcode cache as well as variable cache, like apc.
xcache is popular and famous enough, no less than apc.

APC stores can communicate between each other ?

how can they communicate with each other (account?), can you give example?

it's like all accounts may store php session in /tmp, but it's regarded safe enough - php script can't make use of it to see other account's data.