Can you buy a Red Carpet pass if you're not flying United that day? I vaguely recall that Delta requires you to be flying with them, but could be wrong. Delta's is $50 a day, but the drinks are free

and it is only $25 if you have one of Delta's gold amex cards.
But, the carrier lounges are a good choice. I recently upgraded my credit card to get access to Delta, American, and Continental lounges when I am flying one of those carriers. Pricey, but if you fly often enough, it can be a worthwhile splurge.