This is not a restaurant; it is a living breathing part of Jerusalem history. The restaurant has appeared in songs and poems about Jerusalem as well as many Israeli movies.
Firstly, don't dress up or try to order a table. This is a workingman's restaurant in every sense of the word. Neither are there any waiters, so you'll line up with your tray and order, receive your food and pay before sitting down. Prices are very reasonable. For about $10-15 you can pretty much have whatever you want. For five bucks you can take a plate of humus with pitas and pickles which will fill you up too. The food is all excellent. The humus and kubeh soup are specialties, but you won't go wrong ordering anything.
Try to avoid going on Fridays when the place is very full and also you'd be better of going before 12:30 or after 14:00, it will be quieter.