My 6 year old son is obsessed with Lego. I think it's great to watch him follow the directions to get a big set built. (So far, the biggest he's done solo is ~ 1100 pieces). The process of following the directions, seeing the small changes from step to step, is great for him. Some of the newer sets have some really good engineering behind them too. The above 1100+ set was a Star Wars ship that basically had a Technic skeleton that the rest was attached to.

But I'm far more impressed with the little creations he makes on his own by mixing and matching sets. Usually he makes small transport ships out of anything he can get his hands on, but it's the random stuff that's the best.

(And I think it's hilarious to see Batman fighting along side Clone Troopers.)