It's about the same speed as a simple MySQL query on a single unique key. It's much faster than Cache::Cache and most of the other caching modules out there. It's about half as fast as BerkeleyDB or Cache::FastMmap. It's not even close to the speed of an in-process memory cache.

It is possible to share a cache server between multiple languages if you make both languages serialize in the same way. Memcached just stores bytes, so it doesn't care.

Some large sites use it, so you can probably expect it to be reliable. Check the mailing list for anecodotes and testimonials.