I'm going to add some to this, since I've got the game, as well.
The grinding aspect of the game is definitely not understated. A fact that makes it worse? As of the fourth or so level, the only decent place to buy power-ups is the very first, which means you've got to head back to the first stage and find the shop you want to buy from there, before being able to upgrade. What's this mean? Even though you might earn tons of experience and money as you progress through a stage, you don't see a statistical benefit from this until you complete or leave the stage you're currently in.
Past that, money can't be shared from one account's character to another in any simple way. While it is possible to have someone else on another account take your money, then quit, choose another character on your account, and then get the money back, the task is annoying and takes unnecessary extra time.
Additionally, like Chessa said, character growth can feel a bit excruciating to wait and grind through. The lack of initial abilities for each character means early grinding lacks the variety and style available in the late game, and yes, your character moves very slowly. While I'm all for unlockable moves as your character levels, truthfully, a few more options at the beginning would've been nice, and would've done well to differentiate characters from each other early on.
The game, itself, has several references to the graphic novels, but is virtually unrelated to the movie. This isn't a good or bad thing, but if you saw the movie, it means you'll see some scenes that don't fit align perfectly with it. It's all the more reason to consider checking out the graphic novels, really. Beyond this, lots of content in the game refers to classic video games. When Stephen Stills exits a stage, he leaves via Warp Pipe. Likewise, Kim leaves by taking a Warp Star, from Kirby fame.
Overall, it's worth it for the people who are slightly more hardcore than The Simpson's Arcade Game or TMNT IV: Turtles in Time, in my opinion. The grinding is a major setback, but the gameplay, after that, is a whole lot of fun and pretty challenging.