After my first time at Biteapitta, I was in love! They're falafel is hot, savory and served in a big pitta that can satisfy any of those pre or post going out hunger pains. The humus is great thanks in part to both authentic cooks and Chinese cucumbers. On top of that their staff is polite and not at all clueless, once I was even served by the owner who is a very friendly guy.
My only issue with Biteapitta is that it lacks the cozy atmosphere that I've come to associate with middle eastern restaurants and many other restaurants here in Beijing. It feels somewhat cafeteria like and sterile. They have numerous hookahs (or shishas) but they only have one flavor: apple. This place could really flourish with some redecoration and some more flavors of shisha.
Prices are fair considering portion size (one pitta will do you!), location (if you don't know that Sanlitun's an expensive place by now...) and the fact that it is authentic Middle Eastern food.

Biteapitta gave me a good bite of authentic middle eastern food in Beijing and if thats what you're looking for than go no further!