We got Harry Potter and the Order of the Phoenix before we saw the movie. My wife and I had both read the books, and my kids are still at the age where "spoiler" means nothing.
This was a very well-put-together game. Although the characters' faces left a little to be desired (sometimes the lighting would do strange things to them), it was very solid. I really felt like I was exploring Hogwarts as one large seamless environment. I could've easily gotten lost, except for the ingenious use of the Marauder's Map. You essentially set a waypoint, and footsteps guide you along the most convenient way to your destination, using any portrait passageways you've discovered as shortcuts when appropriate. Brilliant.
The story was somewhere between the movie and the book. If I had seen the movie first, the extra content (little things, like Hagrid's brother actually speaking, to plot points, like the students sabotaging Hogwarts to undermine Umbridge) would've been a welcome addition, making me feel like I'm doing more than just reliving the movie. (As it was, it made me a little more disappointed in the movie in what they left out. ;) )
The music was right, the voice acting from the movie cast was superb, the effects were great. When I finally did see the movie, I was amazed and impressed at how similar everything appeared; I recognized shots in the movie as places I had been in the game.
As far as gameplay goes, it goes back to the single-person play from the first three in EA's series, but it also borrows a bit on the exploratory nature of the fourth. (I'll spare my thoughts on the others for the sake of brevity.) However, it tends to be much simpler than any of them. The game can be distilled as follows: get a task, set the destination point on the Marauder's Map, follow the footsteps there, climb around a bit and/or use spells to move an object from point A to point B. Lather, rinse, repeat. There's very little danger or skill involved, and the game even holds your hand through a lot of the puzzles.
The only difficulty can be finding things when their locations are not explicitly stated. Finding the talking gargoyles, for instance. You're not told which ones talk or where in the castle they may be, and one is on a small balcony that is really out of the way and unseen, such that you wouldn't stumble upon it in normal wanderings.
There is a difficulty setting, and you do get an achievement for finishing the game on "Hard", but there really isn't much difference between "Hard" and "Easy", except for the non-essential minigames and the wizard duels, which you could probably count on one hand and can almost be done by randomly waggling the right thumbstick until the battle ends.
Casting spells is fun and unique. The game does a fairly decent job, most of the time, interpreting the stick movement to the correct spell for the situation. The only time I was a little frustrated is when I was going for the "cast all spells in a single battle" achievement, and for some reason it was resisting levicorpus, casting one of the other attack spells instead. But eventually I cast the spell I needed, the achievement unlocked, and I could get on with life.
Not a very challenging game, all in all, except when it comes to finding things, because the castle is so darn huge. The mechanics are really simple, and the environment is beautifully rendered and true to the source. It was definitely pretty to watch and walk through, even if the gameplay was too casual even for my taste.