My 3 year old son loves these. LOVES. I really can't put it into words how much he loves these T-shirts. It's great because on occasions where we need him to dress nicely, instead of an argument, we let him put one of these on underneath and he pretends to be a superhero in disguise.

As others have pointed out, it runs a little small, and it really shrinks in the wash (more in the length than in the width).  Definitely size up, if not 2 sizes up.