Just read this to my 6 year old twin boys. They liked it pretty well, and maintained interest and enthusiasm over several reading sessions which we don't do much of yet.
For my part, I don't remember whether I had it read to me as a child or not. The book ends with an ambiguous and, to my mind, somewhat sad ending. There is some truly random stuff in this book. An invisible car for one. One inconsistency that bothered me a bit was that it was explicitly stated that Stuart is too small to carry money. In fact there is an episode where he attempts to pay for a busride with aluminum foil coins his father made for him. Later on though he pays for everything from a mini-canoe to gasoline without incident. He even argues with the gas station attendant along the lines that "I need gas and you need the money so why don't you make the sale" when the attendant expresses unwillingness to sell five drops of gas.
I also found Stuarts personality a bit abrasive. He's proud, formal, sulky, and easily offended. Perhaps his struggles for respect resonate more with children though.