Sorry if Mario Lego is off topic, but we already started it.
My 4-year-old daughter and I love it. Hours of play to be had.
The Mario has a camera on the bottom to scan action bar codes and to generally get information about what color Mario is on. Red is fire, blue is water, green is grass and yellow is desert. Fire "harms" Mario and after long enough he gets X's on his eyes. The thing is that anything red or pink gets registered as fire, so Mario can stand on the red roof of his house or even his hat and catch on fire. My daughter thinks it's both odd and hilarious!
If I have a main disappointment, it's that you are basically required to use the Lego Mario app if you want instructions. This can be annoying, because you have to link up the Mario figure and scan a block from the set to get the instructions. I wanted to skip ahead to a specific item in one of my sets and had to go through a lot of hoops like clicking into prior steps and act like I was completing them to unlock the next step. We also moved from one tablet to another and had to "restart" on directions and scanning in sets we already built. The other reason it's frustrating is because I like to keep my daughter off of screens most of the time, so would have preferred paper instructions while we were building the sets.
I guess if I've got another area of improvement to suggest, it's that the action blocks for the specific characters (Yoshi, Toad, Toadette) all result in the same generic reactions from Mario. They could handle this with a software update, but I think Mario should end up playing sounds and animation/icon from the character in question. The bar codes for the individual characters seem to be unique, so I think it could be done, but who knows if they'll do it.