eAccelerator seems to be compared to APC for it's caching + accelerator performance. But eAccelerator has one major advantage to APC and that is file caching. If your server runs low on RAM or your eAccelerator config hits it's own memory limit then the cache can start to store on the disk as well (or choose to store all the cache on disk and leave RAM for other processes).
This is excellent for shared hosting servers / or dedicated servers used for multiple services.
eAccelerator contains a PHP encoder and loader (although the current RC doesn't). You can encode PHP scripts using encoder.php in order to distribute them without sources. Encoded files can then be run on any site which runs PHP with eAccelerator. The sources of encoded scripts can't be restored because they are stored in a compiled form and the encoded version doesn't contain the source. Of course, some internals of the scripts can be restored with different reverse engineering tools (disassemblers, debuggers, etc), but it is not trivial.
eAccelerator stores compiled PHP scripts in shared memory and executes code directly from it, this is a great speed increase as the PHP engine does not have to pre-compile a script before executing
eAccelerator is compatible with Zend Optimizer's loader. Zend Optimizer must be installed after eAccelerator in php.ini. If you don't use scripts encoded with Zend Encoder then we do not recommend you install Zend Optimizer with eAccelerator.