A good foundation for future Harry Potter games that has a few flaws that otherwise hold it back from being great.
Here is what the game will do for you in a nutshell. You will be playing as Harry Potter for the vast majority of the game and have access to your wand and any spells you have discovered in the game so far at all times. As you run around Hogwarts you could Accio some mysterious bricks on the wall to reveal something hidden, Wingardium Levioso a mop to clean the floor, or Incendio to light a torch. Where other games might give you coins and trinkets to find to keep you occupied outside of the main game, Order of the Phoenix lets you clean up Hogwarts and uncover some of its mysteries. All of these discoveries and achievements will also increase your spell power as you gain enough spell essence from them. Gaining spell essence and increasing your power makes your spells more powerful and opens up new content, interviews and such, in the Room of Rewards, which is accessible from the beginning of the game.
The bulk of the game involves talking to the various main students of Hogwarts that film goers and book readers will be familiar with. They often task you with floating a few items, retrieving homework, or helping them repair something. Although it tends to all be fairly simple and in the end not that varied, the appeal lasts thanks to the changes in local, the mix up of spells, and the Wii controls.
For a game based on a movie series that does a rather good job of emulating the stories they are based off of, despite making obvious changes to the plot to keep audiences interested, the game has a distinct lack of story cohesion. Playing as Harry you'll be tossed from fighting off a pack of dementors in that are attacking your cousin Dudley, to a few story flashes with little to no narrative, thrust into Grimmauld place where you'll learn the basics of spellcasting, and finally shunted to Hogwarts. All of this happens over roughly 5-15 minute time period at the beginning of the game. You won't be given the reason for why you are in each area, you won't know why you need to accomplish your goals, and generally you won't have any idea of what is happening in the story at all. Throughout the game other than having the faint knowledge that Dolores Umbridge is bad, Dumbledore is good, and that you need to stop Voldemort, you won't be shown anything more than a snippet here and a snippet there to enlighten you. What this means is that if you haven't read the book or haven't seen the movie yet (which no one has since it isn't out yet) that if you're new the Harry Potter universe you might not know left from right.
Despite the serious lack of story you won't lack direction. There will almost never be a task that you aren't given very specific and blatant directions towards completing. This isn't a bad thing though. With the size of Hogwarts and the fact that what or who you might be looking for could be anywhere, having the game place markers on your map and lead you from area to area via footsteps helps tremendously.
The graphics in Harry Potter are easily some of the best on the system and nearly reach the mark of Final Fantasy XII on the PS2 for quality. Despite the heavy jagged edges that are readily apparent, the world is highly detailed and very vibrant. Hogwarts feels like a magical world with portrait decorated halls, talkative students, and rattling suits of armor. The entire world of Order of the Phoenix is also load free. You can roam from one end of Hogwarts to the other without seeing a single load screen. Harry will occasionally stop running (you can hold a button to run) which is a result of the game loading the area ahead of you (though this is seldom and there is no pop up). The camera can be very picky and will often give you horrible views of the items you need to manipulate with your spells. This can be solved by reentering the area from a different angle or cycling targets with a button press.
Most of the sound in the game comes from the occasional orchestral burst or the shouts of your characters as they cast their spells. A large portion of the actual movie cast lent their voices to the game and the quality is apparent. Even in the cases where the actual actor wasn't used, the voices tend to be fairly close (except for Snape and Hagrid, both of which sound a bit off). There are a few occasions where an odd sound will irritatingly loop as you cast a particular spell or interact with a part of the environment. It isn't as rare as I would like it to be but it is often solved by leaving the area.
The Wii controls themselves are one of the greatest strengths and most frustrating points of the game. Many times it will seem like the controllers in your hands are actual wands and every flick of your wrist will bring forth powerful magic, while other times no matter how slowly, how quickly, or how accurately you move your controllers you won't produce even a spark. I often found that going for Levicorpus (combat) or Wingardium Levioso (Non-Combat), which require you to flick both controllers straight up, would result in Accio, the summoning charm, or Protego, the shield charm, rather than their intended effect. This happen on occasion with the circular spells as well which would occasionally produce the side to side spells or the up and down spells.
Combat in the game as a whole can be frustrating if the controls are acting up. The game makes it seem like every combat is life or death as magic flies crazily about you and at you. It keeps you jangling the controllers around trying to fire off spells left and right when it is much wiser to take your time and try to make sure you cast the spell you want correctly. As far as I could tell you couldn't completely lose any combat in the game, duels or mental duels, though in the combat at the very end I didn't attempt to lose.
Overall, Order of the Phoenix is a mixed bag of tricks. Much like the Every Flavor Beans from the novels, you don't always know what you're going to get. The game can clock in at 6-8 hours if you focus mainly on the story, but even then you'll find a majority of the secrets as they are directly on your path. The controls can be picky and the camera can be a pain. The story itself is also a major concern for people new the Harry Potter world. The game does do many things right though, giving you a beautiful and explorable Hogwarts, fun controls (when they work), crisp graphics, and extra reward content for nearly every action in the game that experiencing at least once, even as a rental, is worth it.
The game is fun and I recommend it as a rental to anyone interested in a great new Wii experience, and I would recommend it as a purchase to any serious fans of Harry Potter. It is definitely the best game in the series and gives a beautiful foundation for the future book six and seven games.