We got this for our 4.5 year-old son for Christmas and it has been a real win. He has a love of construction toys and had recently started building some advanced Duplo creations so we felt he was ready to move on. We chose this set because he really wanted a "Lego aeroplane" but the Lego City stuff seemed too grown up.
This set lets you build a number of different planes and vehicles from the instructions and then you can download instructions for a couple of extra planes from the website. (Unfortunately there is no guide for the helicopters they show in the manual, but an adult could potentially work it out from the photo.)
To start off with, the construction was a father and son activity which kept them busy for the afternoon making all the models, then my son had some time playing with the different items and invariably, pulled them apart over time. However after a few sessions of building with Daddy, he started making them by himself, and now he really enjoys following the instructions and building the models himself as well as making some custom variations of his own.
There are an awful lot of tiny pieces which can be hard to keep track of, but you can order spares from the Lego website. There's also a handy tool which is great for disconnecting tiny/thin pieces that I wish I'd had for my Lego sets back in the day!