I'm a newcomer to the Harry Potter game franchise, so I can't compare this game to the previous entries in the series.
I'm a fan of adventure games in general, especially the classic titles from Sierra and LucasArts, so it was a great treat to have such a large and immersive environment to explore.
Hogwarts looks gorgeous, and the game designers have done a magnificent job making it look every bit like the sets from the Harry Potter films. The castle is HUGE, and you'll spend quite a bit of time discovering all sorts of hidden goodies. The voice acting is top-notch, using some of the actual actors from the film, and the character models do resemble their movie counterparts.
All that being said, the actual gameplay is tedious. Most of your time will be spent running errands. Advancing through the game consists of talking to a character only to discover that he/she needs an item in a wholly different area of the castle. Puzzles are sparse and not terribly challenging -- all you really need to do is figure out which of the spells in your repertoire need to be cast on the available items. For the most part, you'll be using Wingardium Leviosa to move objects around.
The Wii-specific wand controls are fun in their own way, and add to the feeling of "being there" but in the end the actual game isn't that deep or challenging. Replay value is limited, as there's really no deviating from the main tasks, besides hunting down every last hidden chess statue or broken masonry in need of a Reparo spell.
I can only really recommend this to hard-core fans of the Harry Potter books and films. As a game it's not that great, but it's a beautiful presentation of Hogwarts that you'll spend hours exploring.