At work, we're getting an error with APC (which works nicely otherwise) forgetting about a specific class for some pages (but remembering it for others, it just depends which page gets loaded first...). Seems we're not the only ones, it seems Wikipedia has been having similar issues. We're migrating to Turck MMCache, after my initial tests were favourable. For our environment, Turck seems to performs very-very-slightly worse than APC. But I wouldn't base anything on benchmarks on our environment. Hope to be able to reproduce the error with less code and submit it to the APC developers.