We actually have an answer for this one (the best kind of question, we think): sumptuous four-player Puyo-ing. Check out some screens and decide for yourself if that and the wacky new play modes (co-op, a single-player dual-Wiimote mode) are worth the price of admission. Of course, there's another possible motivator for wanting this version over the Genesis version: fear of Dr. Robotnik and his foul machine.
