First of all, we'll focus on the lounges operated by the six largest airlines:
- Continental's Presidents Club
- Northwest's WorldClub
- Delta's Crown Room
- United's Red Carpet Club
- The U.S. Airways Club
- American Airlines' Admirals Club
So, first off: all things being equal, join the lounge that is operated by the airline you fly most often. The simple reason is that not every airport has every lounge, and the airports served by your favorite airline are more likely to have that airline's lounge in that terminal or concourse than they are a competitor's lounge.
That being said, all things are not equal: the Presidents Club is better than the others.
- Complimentary bar service. WorldClubs and Crown Rooms also offer free alcoholic beverages, but the typical cost for these at the Red Carpet Club, U.S. Airways Club, or Admirals Club is $5.00.
- Complimentary wireless Internet access. You'll get free Wi-Fi at the Admirals Club or WorldClub too, but have to pay for it at the Crown Room, Red Carpet Club, and U.S. Airways Club, where service is typically provided by T-Mobile at a daily rate of $9.95.
- Complimentary partner lounge access. All three U.S.-based SkyTeam airlines (Delta, Continental, and Northwest) allow the other two's lounge members access to their lounge whether they're flying with them or not. United and U.S. Airways--the two U.S.-based Star Alliance carriers--don't have such a close relationship and allow one another's members access to their lounges only when they have a same-day ticket on one of the other's flights. What sets Continental apart from its two SkyTeam counterparts is that Presidents Club members also get unlimited use of Alaska Airlines' Board Room; WorldClub and Crown Room members only get that access when flying with their own airline or Alaska Airlines.
Maybe. See, you can use the American Express privilege only when flying with the airline operating the lounge. That means that if you fly into Cincinnati on a Continental ticket, you won't be able to use Delta's Crown Room if you only have the American Express card -- but you could if you had a Presidents Club membership.
Keep in mind, of course, that all of this is subject to change. As I wrote yesterday, Delta and Northwest are in merger talks that may yield a combined airline inclined to change a few things. Moreover, any merger between Continental and United would fundamentally change the alliance structure, and then all bets are off.
For 2008, though, my solid recommendation is to go with Continental's Presidents Club. It's just the best value out there right now for frequent flyers.