Saw it yesterday (6th April) in 3D and it was AMAZING. It was very funny also. They stuck to the actual game for example, to grow bigger...you eat a red mushroom. To be very small you eat a blue mushroom, to freeze something you touch a blue flower and to shoot fire you touch the yellow/orange flower. You also go through the green pipes to go to places and they even showed Mario kart in the film and showed the characters choosing to build their own kart. It was a brilliant children's and Adult film. The colours were so vivid in the film and watching it in 3D was even better I feel.
Definitely one to watch.