I found this one the other day and was pretty excited. Sadly the 'pegs' (for lack of a better word) don't hold the minifigure in at all (they don't 'lock' like a regular LEGO). And the position of the 'steering wheel' doesn't really work for them to sit in and hold on to at the same time. The minifigures do sit in the car just fine, they just don't really lock in, which is kind of disappointing.
Still a cute effect, and I suppose it's a great effort considering LEGO and Mattel are entirely differnet toy companies, but they could have done a better job assuring the minifigures fit tightly.
Not having any to test with it, I have to wonder if maybe the car works better with a Mega Blocks figure. I find it odd that they even named the thing "Let's Go" when Mattel has had a deal with Mega Brands for a while (and now they even own them) for Hot Wheels and Barbie themed block sets.