For interiors it's a piece of piss but if you do any exterior work it can be a nightmare, especially if you happen to build something of a reasonable size and cross into another cell. The follow AI is pretty ropey anyway when you're outdoors but it's really annoying when I have to keep Fus Ro Dah-ing my follower over the Navmesh cracks!
Hrm, I didn't have a problem with it, but I have spent quite a while watching over a 3D modeller's shoulder. I did find the whole construction set thing remarkably tedious to use though, so you're probably besting me in the patience stakes.
Navmesh is a fucking pain in the arse. Eight frigging hours I've spent trying to get it to work properly and it still won't match up its edges properly.