This is the best co-op game I've played with my son (he's 7 years old). We've completed the main story line in 4 or 5 play sessions, but we're still going to back to find some more hidden stuff, unlock new characters, and generally have fun smashing lego bricks.
In terms of the Lego series, we strongly preferred the Indiana Jones environments, it's just more colorful and interesting than the sterile Star Wars spaceship scenery. Before playing through, we watched the first 3 movies on DVD, I think this added a lot to the experience! Probably even helped to solve some of the puzzles.
Overall, great fun! Right now we're looking forward to Lego Batman, I'm hoping it's not going to be too dark, that's my main worry from the previews.