The M's setup is manageable at best and makes the most of the touch screen's size and functionality. At the same time, the setup is clunky. Touching a piece allows you to move it anywhere on the screen. Swiping up and down on the screen with another finger rotates the piece 90 degrees. The setup sounds great, at least when it is explained. However, it isn't incredibly comfortable and slows your progress down considerably. If one finger lifts off the screen, the piece drops. Additionally, using two fingers can sometime obscure the piece (usually in Medium and Hard), forcing you to drop it, and then pick it up again. Sometimes the screen doesn't read the swipe, so it doesn't rotate. Time limits for some puzzles are harsh, and even when pieces are easy to place, the interface gets in the way.
Control issues aside, The M isn't a terrible game, and shouldn't be considered a "waste" of money. At the same time, it lacks the same replay value found in other mobile games. There's always the opportunity to try and beat your original time, though you'll rarely feel drawn to do so.
For the price (99 cents), The M is worth a purchase if you like jigsaw puzzles.