One of the perks of top tier TK status is owning a TK *A Gold card, which allows access to any *A lounge in the world, regardless of which *A airline you are flying AND regardless of where you are flying (domestic or international; see old 2012 TK status match thread).
One caveat: I’ve personally entered a few UA RCC’s and also a US lounge with my TK Gold card, but recently thought I would be able to enter a *A lounge at DFW even though I was not flying a *A carrier at the time. It turns out, at least in the DFW D-Gates Lufthansa lounge and in the DFW C-Gates RCC, you have to be holding a valid *A BP.
Nevertheless, for domestic *A travelers, a TK *A Gold card is a big deal because a *A Gold card issued from UA (or US) does NOT allow you into a domestic *A lounge like the TK Gold card does, unless you are flying internationally.
BTW, this is not the case for the RCC Club membership. With the RCC club card, you can enter a US *A lounge (past security) even if not flying anywhere.
FYI, the policy specific to these rules is here:
http://www.staralliance.com/en/benef...access-policy/
Star Alliance Gold Customers Travelling in Any Class:
Customers have access to any Star Alliance member carriers' owned lounges with the Star Alliance Gold logo at the entrance.
Customer must present proof of Star Alliance Gold level status via a valid frequent flyer program Star Alliance Gold level card or other valid indication of Star Alliance Gold level status
Customer must also present a boarding pass for travel on a Star Alliance flight departing from the local airport
Customer is entitled to one guest
United and US Airways Star Alliance Gold customers may only access the United Clubs and US Airways Clubs within the U.S. when travelling in conjunction with a Star Alliance international flight.
That means other *A members in the US can access the clubs even traveling domestically.