I have to say this is probably my favorite purchase of the year.  I've never owened a union suit but I don't think I'll ever go through another winter without one.  Wearing it underneath clothes, I hardly know I have it on!  It keeps me warm when I'm outside and keeps my heating bills down when I'm inside.  It's like a second layer of skin...and there's a butt-flap for when I need to poo so I don't even have to take it off.  Get one.  You won't regret it.