This was probably the only on-a-budget eating place I really liked in Budapest during my 3 day stay there. Generally speaking, the service in Budapest was much to be desired. At first I used Lonely Planet Central Europe '99 guidebook, but their top choices were so bad that I had to put it aside.
Marxim was recommended to me by a local Budapest guy, who incidentally was my guide for caving in Buda Hills. Marxim is located in Buda, a walking distance from Buda Castle. This place is really cool. It has communist banners and pictures of Lenin and Stalin on the walls, iron and barb wires separating the tables, and a lot of other funny stuff pertaining to the communist years. Besides the atmosphere, the food is pretty good. Each dish has a name that is somehow related to the communist theme, like Stalinist pizza or Trotsky's breadsticks (I don't remember exact names, but you get the picture). Of course the description of what is included is listed next to the item.
Marxim's location is not on the tourist path, and therefore the service was good. What's more, the prices are very low by Western standards, so this place would be perfect for a budget traveler.
Overall, Marxim's combination of good service, good and cheap food, and a truly unique atmosphere makes it an enjoyable place to eat.