After much trials and tribulations, Wikipedia is finally using APC. They've been playing around with Turk MMcache and other accelerators for a while. But currently APC is the only one with the ball as far as caching is concerned. Recently, somebody did a benchmark on the common accelerators used in php land - read it here. But at that point APC just wins hands down, though my commit last night probably must've pushed APC below eAccelerator, it is required to run properly on a multi-CPU apache on high loads.
Now, the next heavy user of PHP around is sourceforge.net who is apparently still using eAccelerator. Apc still has a few chinks in its armour, but it is still *my* work. And much more importantly it, for once, doesn't appear doomed :)--
Real programs don't eat cache.