On Linode I was getting jammed on I/O; on DigitalOcean I am jammed on CPU.
I'd be happy to give up half my RAM to get twice the cores, actually. I run a modest Wordpress network (http://ozblogistan.com.au). Once MySQL is humming, most of the CPU time is spent on various copies of PHP running the increasingly bloated Wordpress codebase.
(Yes, I've used opcode caching. I've never had good experiences with any of the major ones).
Just wondering, what were your experiences? Installing apc is basically apt-get install php-apc (if you're on Debian/Ubuntu), and that's about it unless you want to increase the memory limit. This will greatly reduce CPU usage.
I periodically try again.
By the way: Starting with PHP 5.5, Zend's own opcode cache will be included by default.