Right now, I'd put his appropriate value at high 2nd round. He's not a RB I'd look to for carrying my team, and if I can think of 12 players I'd rather have first, he's not 1st round. In no particular order:
LT, LJ, Jordan, Alexander, Tiki, Edge, Rudi, Manning, Portis, Jackson, Westbrook, and there are a lot of guys whose situation is pending I'd consider over McGahee.
Dunn outperformed him, but 1st round would be jumping the gun. Thomas Jones I probably wouldn't take because of Cedric Benson looming the background. If Bell or Anderson for Denver ever got complete reign over the running game, they'd be above Willis. Julius Jones, Ronnie Brown, Caddy, and FWP all have the possibility of jumping ahead too pending team changes this offseason.