I really enjoyed this game, having played lego star wars, indiana jones and batman this is now my second favourite lego game.
The game play is intuitive and can be played in story mode and free play. There is a fun way using the DS stylus to cast spells and mix potions. Each year you complete you learn more and more spells and these are then used in free play to unlock hidden areas. There is the standard format of collecting studs and wizard hats (the new canisters), red bricks and this time trading cards to unlock characters who can then be bought. There are of course the classic lego clips which are funny in a way that only lego can manage.
The only down side to this game is that there are no mini games. Also you need to use the stylus to make the characters jump which can be a little frustrating if you are used to using the A and B keys. For this reason the game loses a star.
In summary this is a great game for the DS and I will be on the look out for lego harry potter years 5 - 7 and I will be pre-ordering it.