Don't be swayed by the critics on this one! The Mario movie is great, from start to finish. I've been playing Mario since the 80s, so this review is coming from a total fan of the series.
The critics like to write off the movie as simple "fan service," but it's deeper than that. It's important to remember that Super Mario is a video game series based on a platforming mechanic - it's the game that defined the genre. And the movie did a perfect job of giving that same feeling of playing the game. I got to see Mario trying to jump from one platform to another repeatedly - failing over and over and then eventually succeeding. THAT is what Mario is all about, and the movie is a beautiful homage to that feeling of holding that controller and trying to will Mario to success. The "I Need A Hero" musical montage brought a tear to my eye because it captured it so perfectly.
The movie was fast paced, a total visual feast, and there was just enough of a story there to hold it all together and to make me feel and root for the characters. The movie didn't try to take the classic story of the game and add multiple layers of complexity and back story. Instead it kept it simple and focused on that awesome videogame-y feeling, and I totally appreciated it.
If you ever stayed up late into the night playing a Mario game, you will understand. The Mario movie is above all else FUN!