The alfa wireless adapters work very well with the latest backtrack. The 51n are dual band but seem to be out of stock everywhere. The 36n also works well but it is 2.4 only.
Problem with these adapters is your general consumer say ...oh they have big ugly antenna and have long USB cords...I want a tiny thing that plugs directly into my USB port.
I feel these are advantages but us techy types are losing out to the sheep who buy what every has the most pretty flashy lights.
Alfa tends to lie a little many times you will see models with different watt ratings, all that means is it has a different antenna.
From a general use they work about the same as any USB stick of course with the added "advantage" if being able to place them on top of your computer because of the USB cable.
From a backtrack use I would buy a 19db flat panel direction antenna and a adapter that lets you directly attach the alfa to the antenna. These antenna are about 15in sq and are about the best you can do for range...unless you want to use 24db outdoor antenna but they are huge. Both can be had for less than $50.
Note you will need different antenna for 2.4 and 5 bands.