The package also contains a useful status script, just copy the shipped apc.php to your document root. You'll see how the usage of the SHM segment grows by time, and after some time you'll also see what settings to use for apc.num_files_hint and apc.shm_size when the whole site is cached.
Two weeks ago I plugged APC onto my main customers site, and I'm really satisfied by it now. I already tried it some time ago, but back then it had some problems with PEARs Quickforms and similiar heavy OO code, but those problems are fixed for about 90% now, some 8% can be easily fixed by reordering require statements and the like and finally the remaining ~2% are going to be fixed by Rasmus in the foreseeable future.
The APC package is "the Alternative PHP Cache. It was conceived of to provide a free, open, and robust framework for caching and optimizing PHP intermediate code."
