For me, they provide reliable wifi, space to work, clean bathrooms, a sometimes badly-needed shower, and equally badly-needed preflight or post work cocktail. I travel on business a lot and am based in a small city where connections can sometimes be long and multiple. This makes membership a necessity for me.

Some of the clubs are outstanding - DCA, DEN, SFO and LGA have great locations, same with ORD H/K lounge, and there's a range of options at DFW where I typically connect. The AS lounge and 2 AC locations at LAX are definitely adequate, and the N satellite AS lounge at SEA is one of the most impressive domestic lounges anywhere (except the frustrating lack of showers). If your use is at places like PHX, CLT, PHL, MCO, it'll be rather disappointing.