The UA RCC in the C-D walkway is small and nothing particularly worth going out of your way for - not bad, not great. I haven't been in there in a while, but last I was it could've used a refresh of the seats. If you want to relax during your layover, just go to the US Club in F. While smaller than the B/C club, the service is pretty much equivalent and you'll be close to your connecting gate.

If departing PHL on a transatlantic flight on US or LH either in business or as a *G, definitely check out the Envoy Lounge, but it's only available in that case - not with a connection to a domestic US flight.

The best lounge in PHL is the BA lounge, however.