This game is a pretty standard 3rd person shooter. It's fun, especially for fans of the Harry Potter movies, but it doesn't offer anything innovative, and it's rather short.
Each level involves you taking control of one of the movie characters (a wide variety of different ones) and working your way through a level that is crawling with death eaters. You choose your "weapon" using the d-pad (stupefy, impedamenta, expeliamus, etc., etc.), aim by pointing at the screen with the remote, and move with the joystick. Each spell is more or less analogous to a gun type that you would find in a typical shooter. (Stupefy is like a pistol, expulso is like an automatic, petrificus totalus is like a sniper rifle, etc.) The combat style emphasizes crouching behind cover, and taking out your enemies before they can flank you. A couple interesting spells you won't find in a shooter are protego (which allows you to shield yourself while out in the open) and being able to apparate around the battlefield.
Just to mix it up, there's a level here and there where you have to do something else like run away from a danger before it catches up to you, snipe enemies from a bridge, or protect a teammate who has an exposed position.
The gameplay mechanics all work perfectly fine, and the game makers do a decent job of keeping it from getting repetitive.
On the negative side, there's really nothing special about this game, except for the harry potter theme. Also, I was a bit disappointed in the end part of the game. The final battle with Voldemort is uninspired, and you don't get to fight nagini (the snake). In fact, you don't get to kill Voldemort, as after you duel him, the very final part of the game is a hokey mini-game rather than a battle. This was pretty anti-climactic.
Conclusion: I'm happy I rented this game, as it was fun for a couple of days, but I'd feel ripped off if I bought it.