All the people that say "stay off MUPs" must only have really some really leisurely ones. Out here there are sections of the longer ones where you'll see people on their Sunday ride, and some of the shorter ones have lots of runners/walkers, but there are some that stretch pretty far out to where you'll only really see "cyclists".

I'd still consider it a silly place to do intervals or some other intense training, mostly like to incorporate them into longer rides. It can be relaxing to get off the road when you're on it for 5+ hours.