For what its worth, with the Red Carpet Club membership, you also gain access to any of the US Airways (/America West) clubs when flying on US Airways.

But as the previous poster mentioned, it really depends on where you are generally flying. It can be quite inconvenient, and sometimes nearly impossible, to get to another airline's club depending on where it is located in the airport, what terminal its in, whether or not the terminals are connected within security, etc.