E.T.: The Extra Terrestrial is widely regarded as the worst video game ever. I disagree with that characterization somewhat. Yes, it is certainly true that it is not a great game and is also an extremely frustrating game. However, there were many worse games before and since.
E.T. gets this reputation for several reasons. First, it was a heavily advertised, highly anticipated game that was also based on an extremely popular movie so the associated disappointment was that much bigger. Second, it is easy to attribute the video game crash to this game. This is an exageration but it is representative of what was wrong with the game industry at the time (too many games rushed out too quickly, sacrificing quality) and coincided with a huge downturn in video game sales. E.T. sold about 1.5 million copies which was reasonably well by the standards of the day but far, far less than what was anticipated. Many cartridges were ultimately buried in a landfill in the desert (and later found, but that's another story). Finally, the game WAS pretty bad. But given that the developer had five weeks to complete it, it's amazing that it is as good as it was.
From what I've read you really need the instruction manual. Most of these Atari 2600 games were very cryptic especially with graphics of the day.
5 weeks is impressive to get something working and out the door considering the pressure he was under.