I'm an adult who only recently rediscovered Lego, and found this set to be great fun. It took about three hours to build, non-stop.
You can steer the truck using a little gear on top of it, which is a nice touch. Cabin doors can be opened and seats can be reclined. The container can be inclined or fully unloaded, using the motor; you have to be careful to switch between modes when the mechanism is in the "rest" position only. One minor criticism I have is that it's very easy to make the actuator overrun, which means you'd probably break it when using it a lot; it would be nice to add some mechanism to automatically turn off the motor (which, by the way, requires 6 AA batteries, not included). I bet you'd only need a few additional pieces to link motor an wheels, making it a creative challenge as well.
The alternative "Tipper Trailer" building instructions must be downloaded from the Technic site; bizarelly, the first part is downloaded as a simple pdf file while 2nd and 3rd are zipped.
All in all, it's a really fun kit.