Personally I prefer the club on the B concourse where you go down to the same level as the ramp. It's the newest club for UA at ORD and has been open roughly four years but still has a fresh feel to it compared to the tired (but larger) C club.
Some threads from the UA forum:
http://www.flyertalk.com/forum/unite...ited-club.html
http://www.flyertalk.com/forum/unite...-now-open.html
Other resources:
http://www.united.com/web/en-US/cont...e/default.aspx
http://www.loungeguide.net/wiki/u/Ch..._*_Concourse_C