The problem with these puzzles is that you never know if it's actually solvable until it's solved. The author of the puzzle could store any number of bitcoins in some wallet, publish a difficult but unrelated puzzle with the claim that the solution reveals the private keys to the wallet, then laugh while the internet goes on a wild goose chase. It's even more likely since the author is usually anonymous (obviously for good reason). They're fun to ponder over a little, but I'd never invest more time (or money) than I could afford to lose into any of these, no matter the prize.