My 5year old son loves our Wii and loves Mario Kart, Wii sports, and Super Mario Brothers 3 (From the NES system and bought with Wii points on the Wii) We also have a Spongebob game for him, but that can frustrate him pretty easily. All that being said, it is not his Wii, I actually gave it to my wife for her bday. The adults of the house get way more use out of it than the kids. So unless you will be using it as well, or anyone else older in the house, that is a lot of money to spend on a 5 year old. While he loves the games, for most of them there is just a wall which he chooses not to pass.
I also think the Wii is great for motor skills. Since my son has started playing the Wii, I have noticed a big improvement in his hand eye coordination. Also in his problem solving skills. I am strict though about how he plays it. When he is actually trying to advance in the game, I let him play. But once i notice that he is just playing simply to be in front of the TV, I make him turn it off.