LEGO Star Wars II is, simply, a very fun game that can be enjoyed by a wide age range. I have the PC version, so this review is specifically based on my experiences playing the PC version (using a PS2 controller), but I expect that the game plays similarly on its console ports (GameCube, PS2, and X-Box).
The game plays through the plot of the Star Wars original trilogy - Episodes IV through VI - using LEGO pieces. The characters are all built out of LEGO pieces. Every piece is a real LEGO piece, although quite a few come from the Star Wars LEGO sets. (For example, the lightsabers, C3PO's head and Darth Vader's helmet.) You could imagine the game playing out in reality out of LEGO parts, although a little imagination would be required to make the blasters work and ships fly.