Turn left at the top of the Roppongi hill (away from the elevated freeway and towards the McDonald's). It'll be a few hundred feet down on the left. Big red sign and rotisseries spinning. Can't miss it!
You'll be pleased with the prices and the food is very good. I had not had a beef pita sandwich in that style before - it was a unique sauce, but still it was very flavorful. The sandwich and the beer was 1000Y. It's also a great place to duck into if the Nigerians become unbearable.
The sauce was good, but if try it first to see if it suits you.