Let me just say this now to begin with. The game itself is definitely worth playing. It was a fantastic title and a fitting piece of media for those that played the other two titles. I'm giving it a no in recommendation because of the ending.
This is similar to Mass Effect 3. The creators' desire to have this out-of-the-box ending ended up ruining my love for the whole franchise. Instead of giving us the tv tropes ending we were hoping for, they decided it'd be better to have this open-ended depressing conclusion that makes you want to eat fecal matter to wash out the taste.
Should you play it as a fan of Deponia? It depends. Are you playing it for the humor and the puzzles and don't really care for an ultimate happy conclusion? If so go for it. If you're like me and got invested in Rufus' and Goal's story, then do yourself a favor and ignore this.