Persistent caching using APC, xCache, eAccelerator, Zend Disk Cache, Zend Shared Memory Cache or files.
The plugin implements object level persistent caching and can be used instead of the built in WordPress
Unlike WP Super Cache, Hyper Cache and other plugins, EM Object Cache does not cache the entire page; instead, it caches the data WordPress explicitly asks it to cache (using
wp_cache_xxx() API functions).
Although this means that the performance will be less than with, say, WP Super Cache, all your pages remain dynamic.
EM Object Cache won't help you much if the plugins or theme you are using do not use WordPress Cache API. This is by design, since the plugin tries to play nice. However, for most WordPress installations this will not be critical.
EM Object Cache significantly reduces the load from your database. Say, my blog's home page without the plugin executes 24 queries (0.02403 sec); with the plugin enabled, only 4 queries (0.00188 sec). Unlike DB Cache/DB Cache Reloaded, the plugin will work in the Admin Panel and supports all plugins that use WordPress Cache API.
CAVEATS: * if you want to use FileCache caching engine, chances are that the plugin will not work when PHP safe mode is enabled and web server is operated by a different user than owns the files.