The Halal Guys are actually going to be opening a chain of restaurants similar in idea to Five Guys Burgers. They are going big time.
One opened in Las Vegas (in Chinatown) late last year, its ok I think most bigger cities have local gyro/falalfel/schwarma places that are going to be far superior. There are at least 4-5 other med/greek places in Vegas that serve better food at similar prices and I imagine it'll be the same in LA, SF, Chicago, Dallas, Atlanta, etc.
What made the halal guys special in NYC is the price, high food turnover (so it was always fresh) and location (cart), in most cities the price advantage just isn't going to be there.
The great thing about the vegas spot is its hours, its open 10am - 4am every day. Yup, hungry and drunk at 2AM then fuck ya a gyro or chicken & rice is fantastic.