In 2009-07-07 was released the first stable of memcached, the php library based on libmemcached.
Now you have two options (in php) memcache or memcached.
memcache is the usual one, but memcached promise many improvements over the other one: one of my favorites are fact of use libketama for keys which allow sharing keys between languages (like Ruby, Python, etc.)
so, did you have a chance to use/test it, do you feel is it ready for a live enviroment?
Edit: the reason I ask this is because move from one driver to other means a great effort that I’m not sure to yet.
The old C library libmemcache.so had some serious bugs and leaks. The new C library libmemcached.so, which is the basis of PHP memcached, was written specifically to not have those bugs and leaks, and also has significant performance improvements. It is also under active maintenance.