We use a slightly more versatile toy--Math Cubes--as a substitute for Lego with DS2. He's managed to figure out number relationships by building things with the blocks. He also loves tesselation tiles for building and using in pretend play, and he's picked up how the shapes correspond to one another indirectly through his play.
Like ColinsMum, I'd say try it if you think your child will have fun exploring with it. I'm a big fan of making media available, sitting back, and seeing what my son does with it.
