PHP opcode caching has always been a hot topic here in the forums. Check our wiki and forums for APC how-to.
Just one of LiteSpeed's client setup:
1) PHP 5.1.4 everything.
2) Over 1/3 billion php impressions per month.
3) Opcode cache: APC.
Result: 0 error logged as result of APC.
Other reasons you should consider APC for PHP 5 codebase vs others.
1) Support. The developers are heavy pushers in PHP/Zend world.
2) Support. Bug reports are handled extremely fast, like LiteSpeed.
3) Support. Yahoo reportedly use APC. That's gazillion test cases.
4) Support. Get the trend here? =) Code compatbility with latest PHP 5 base is second to none.
If you found a bug within APC. Report it via their pecl.php.net section.