They are a quad with two missing wheels.
I think they are totally fun.
Yes, they are odd, heavy, slow, low, and most of the other bad things people say about them. However, they are great fun.
Ever ridden a KLX 110 set up for an adult or a XR80 set up for an adult? Like a TW200, they suck compared to a "real" motorcycle. But like those, the TW is totally fun, and in a different way from those.
If you are buying a TW as a novelty, I think you'll be happy. If you are buying it for commuting or motocross or Moto GP you are likely to be disappointed.