My son is a big fan of toys from a building set manufacurer based in Denmark, due to international copyright laws i can't mention their name, but it starts with L, sounds like an egg in the middle, and ends with O.
Anyway, we also have some of the K'nex "Lego style" sets also. Whoops I said it. Compared to Lego, this is what I would say about them-
K'nex are good quality, and fit well with Legos.
The bricks are slightly chamfered, which makes them not look as good on a model but easier to get apart.
The axle type pieces are a different diameter than Lego, so they aren't completely compatible.
They seem to hold up well over time. We haven't had any pieces wear out.
K'nex has some cool plastic spring pieces for suspension in a lot of their models. No idea if any of these do.