It was a terrible car in some (many) respects, but it definitely is not "the worst car ever built". The Trabi has a lot of things going on for it.
1. It is very light. The motor is tiny, the whole thing is like 600 kilos (not absolutely sure, but I know for a fact that 4 normal men can pick it up to move it to the side a bit).
2. Given its size, there is quite a bit of space inside
3. Since it is so small and light, it is very easy to drive. You literally don't need the clutch: there is a clutch pedal, but you don't need it -- you can switch gears as long as you are not pressing the gas pedal. (The gears, btw, are very light, and the "stick" is not a stick, it is on the wheel -- you can operate it with one finger without taking your hands off the wheel.)
4. With electronic ignition (it can be installed easily on top of the stock model) the motor behaves quite well.
5. It is cheap, not only because it is bad, but also because it is just cheap. Not many moving parts, nothing fancy in it, you get the point. I don't know how accessible spare parts are nowadays of course (this was the problem back in the day as well; finding spare parts was a bitch).
It isn't a "good" car, given what we expect from cars these days, but for a city car it is quite OK. Still not as dirty as a diesel ;-)
PS: there is an old Trabant joke that goes like this: a car collector from the US hears about it and wants to have one. He orders it and has it shipped over. On arrival, he looks at it in confusion, then writes back to the sender: "I wanted the car, not the toy model!"