Unionfs - is it actually usable?

[Arthur Chance]

Ever since RELEASE-5.0 man mount_unionfs has contained a section saying

BUGS
	THIS FILE SYSTEM TYPE IS NOT YET FULLY SUPPORTED (READ: IT
	DOESN'T WORK) AND USING IT MAY, IN FACT, DESTROY DATA ON YOUR
	SYSTEM. USE AT YOUR OWN RISK. BEWARE OF DOG. SLIPPERY WHEN WET.

On the other hand, I've occasionally seen posts here from people who use 
it. So, is unionfs usable, and if so, under what circumstances 
does/doesn't it work?