As a Harry Potter fan, I loved
Harry Potter and the Order of the Phoenix; as a gamer, not as much.
The entire game has a great, what I like to call "Dark Ride" feel to it. Most of the game is built around exploring the castle and discovering things to do as you progress through the plot of the movie. There are lots of little things scattered throughout the castle that fans will love. While digging around the Defense Against the Dark Arts classroom, you'll uncover references from nearly every movie; a blue imp who is still trapped in a desk, Lupin's movie projector and even a left-over Gilderoy Lockhart poster. You can also use spells on brooms and mops to clean up the school, take practice O.W.L.'s or collect passwords from the various portraits. So, for fans of the series, the game is the next best thing to actually being at Hogwarts - at least until the recently announced theme park opens.
From a gameplay perspective, the game really isn't that much of a game. I had fun, but again, I am a Harry Potter fan and I like these types of immersive experiences. When you get below all of the fan service, gameplay is pretty shallow. Narrative is the driving force behind the entire game and the only thing that pushes you along. Most of your time is spent finding students to join Dumbledore's Army, most of which require small fetch quests or other tasks. A few puzzles pop up in different areas, though they are very easy to complete and very light on challenge. Wizardry duels, which should be some of the more challenging and exciting areas of the game are exceedingly easy. Of the six or so duels in the game I lost one, and that was because the story required that I lose it. The others, including the duel with Voldemort, are either no lose situations or I'm a better wizard than I thought.
When you aren't exploring the castle or completing plot-based tasks, you can also seek out mini-games like Exploding Snap or Wizard's Chess. You'll find students playing these games throughout the castle who you can challenge. As with discovering secrets, you'll earn exploration points. These act like experience and level up your magic (which may be why I dominated in duels) and unlock treasures in the Room of Rewards. The integration of unlockable rewards with the main game is phenomenal. Rather than having to exit to a menu on the title screen, it is all stored in a room in the castle; all you have to do is click on a treasure and you are presented with your unlocked item. Some of the earlier rewards, which are interviews with the cast, are better than some of the later ones and present a nice background for anyone interested in what goes into making a game.