Jeff,
I buy the coveralls big to use as a top layer and they work really well for me. I'll have 4.0 and regular layers underneath. The only thing I don't like coveralls for is walking as I get way too hot. My current Walls set fits in my pack, so I only put them on when I'm sitting somewhere for a bit. That said, I have run the coveralls alone with no ins base layers when I was doing stalking in cold weather. It worked well, but I wouldn't have wanted to sit long w/o the layers underneath(Usually throw a light jacket in the pack just in case).
I think the coveralls help more in the wind and keep the heat in much better than the bib/parka combo since they are one piece.
Cool. I'd look into it again if the bib/coat thing wasn't working so great for us! I haven't put on a coverall, other than fire coveralls, in so long I have no clue.. likely the 90s...