We have changed the site server caching all over to Xcache.
Previously we used a combination of EAccelerator and memcached.
We are opcode caching the compiled PHP code and data caching a lot of static vBulletin data in a shared memory pool.
Our monitoring indicates a small performance improvement over the old method.
Here is a summary of performance since last night. (Lots of hits/s is good - each one is a disk/sql access saved)