I tell ya, my boys love the Legos. But they treat them like models. They don't play with them because they tend to fall apart. They enjoy putting them together and displaying them.
They also really like Bionicles. They can play with them (although Matthew has lately grown out of them...he still enjoys putting them together). They don't fall apart so easy.
Luke loves Legos and Bionicles. And he really likes K'nex as well.
But he ~does~ play with them. We've got parts scattered all over the house and he wonders why we can't put them back together.