FWIW, I played and owned a lot of high-end basses over the years, and I'm in total agreement with all those who say that the Thumb is one of the best-sounding basses ever. It's a unique tone that makes a lot of the competition sound gutless by comparison. I think the five string version sounds even better because of the slightly different neck pickup placement, maybe.
However, I'm also in agreement with those who can't live with the ergonomics. What's a comfortable and easily playable bass varies from player to player, but it's fair to say that the Thumb is very much a niche product when it comes to playability. If you're a big guy(or gal!) with long arms who does weightlifting then your going to fare better with a Thumb than a five foot five weakling who is used to playing a Fender-style bass. I can play a Thumb bass sitting down okay,but standing with the bass on a strap I struggle. That's why I haven't got one.