Thomas, I do not have much experience with PHP 4.X as I have moved everything to the 5.1.X branch so my feedback regarding APC will reflect that angle. It's been a long while since I touched PHP 4.X.
APC 3.0.10, latest release is rock solid, there are clients using LiteSpeed + PHP 5.1.X with APC 3.0.X with about 10 million page views per day and there has not been a single core-dump or error logged as result of APC for months. Of course, this is with an PHP 5.1.X optimized code base.
However, there is one caveat.
If you are using APC with something like vBulletin 3.5.4, you might encounter 1 or 2 scripts that give fatal errors under APC and none without APC. There appears to be unresolved bug in APC's PHP object handling code when the code is written based on PHP 4 standards.
vBulletin 3.5.4 uses mostly PHP 4 based object conventions so for these type of rather bloated programs that strive to have the highest PHP version compatibility, you might run into legacy problems with APC where the most recent efforts have gotten into PHP 5 compat. Once I get to reproduce the problem again I intend to file a bug report the APC devels.
Basically, if you develope and run your own programs, APC's stability and performance combination cannot be beat and you can always work around bugs. If you run 3rd party bloated programs, then you need to test it out and make sure everything works.
That's the best advice I have regarding this issue.
Because of the level and quality development that has gone into APC in the last year, LiteSpeed will likely standardize on APC, as part of it's add-on install option, in the very near future.