It can be kind of... boring.
You play with them for a while - say 20 minutes - and it is wonderful. You see their little minds grasping new concepts, new pathways being formed. You see them making connections. It was Aristotle who said that people learn by imitation. You see the odd bits of adult behavior that they pick up. My son just constructed a mock-up of my office out of mega-blocks. It included my office, my boss' office (I like to bring my son by so my boss will take pity on me and continue my employment), and the elevator we take to get up there. It also had the office of the guy who has a plastic samurai sword. Oh, and he included the bobble-head bear that is a centerpiece in our conference room (don't ask.)
However, children require repetition to learn. The first couple of times we play "office" with Legos is pretty great. It starts to wear. If you love your kids, but after an hour of this sort of thing you are desperate for adult company - you are normal.
With that in mind, I will soon launch a series of entries on boredom antidotes.