I haven’t tried it on WPMU myself, but I’ve heard my XCache plugin works there – I won’t swear to it though 🙂
I wouldn’t expect there to be much of a difference in the actual page load times under no load between Memcached and XCache – they both work the exact same way, after all. However, the Memcached backend (as a result of its intended usages and language-agnostic facilities) will slow down once the load starts – but that’s just my experience.
Please do let me know how it goes with WPMU, I just don’t have the time to test it out! (sad, I know ;))