I'm enjoying Puyo Pop Fever for the GBA right now, and it's really good, but quadrupling the speed mid-game does not bode well for gamers who want to complete it. I lost 25 times in a row to the final boss at the end of the Waku Waku Course because of the speed alone. Still, it's a pretty solid and fun puzzle games with over-the-top character designs and a charm that definitely nails its target market pretty well. If it weren't so difficult, though, it would have attracted a much wider audience, which is a real shame.
Personally, if I were in charge of the difficulty of the game, I would have just made the AI smarter and smarter as you get through it, and the maximum speed increases would be only double speed instead of this funky quadruple speed increase the game's got going on. Overall, it's challenging and fun, but sometimes borderline frustrating.
9/10 for Puyo Pop Fever
So what are your favorite puzzle games (excluding Portal)?