Some recommend XCache over APC for performance, personally for me I use APC as it is now part of PHP and maintained by the same folks. In all honestly I have not noticed a difference (and I used to use XCache all the time).
In this article, the author concluded that xCache is faster and more stable.
I agree with him, xCache is faster and less bugs.
In the second run though both xcache and apc have the same transaction rate of 1.53 which is 23% higher than that of no cache 1.24, but the longest transaction and shortest transaction of xcache was better than that of apc. It shows that xcache handles caching better than apc.
Eventually, there were some bugs in apc caused crashes when i tried to implement it. And xcache ran fine without any issues. This scenario shows that xcache is better.
In my setup eAccelerator ended up being about 10% faster than APC/XCache. But even so, I uninstalled eAccelerator when they decided to remove a bunch of features I used in their newest version.
Previous eAccelerator versions also provided functions for use in PHP scripts that allow access to shared memory, automatic web (content) caching, and other related tasks. These were removed as of version 0.9.6rc1.
Seems like it was a good choice to remove it these days... development has more or less stopped.
I think the thing that made eAccelerator a little faster was because it had not only an opcode cacher, but also a code optimizer.
Currently I use XCache (I used APC before I switched to eAccelerator). I've been tossing around the notion of going back to APC since it's a "little more official" (if there is such a thing) with PHP devs working on it. But it's also one of those things where if it's not broken, don't fix it.
I'm using xCache as well and haven't had any problems. However, xCache was made by the developers of lighttpd, benchmarks show it performs similiarly to eAccelerator and faster than APC. Either way APC is handled by the same developers who work on PHP, so that might have something to say for it.
Alternative PHP Cache is a free, open source (PHP license) framework that optimizes PHP intermediate code and caches data and compiled code from the PHP bytecode compiler in shared memory. APC is quickly becoming the de-facto standard PHP caching mechanism as it will be included built-in to the core of PHP starting with PHP 5.4. 
5.4 B1 did not bundle APC so it looks like we'll have to wait for it...guess they prioritized other things (look at all the syntax changes!). Maybe they thought APC isn't stable enough yet? I installed it on my server and if I don't allocate more memory than all my scripts combined use, apache starts segfaulting (w/ APC 3.1, 3.0.19 was fine but it doesn't support PHP 5.3.8).