My four-year-old is thrilled with this toy! Due to his age, he initially needed parental supervision and guidance to build anything, but once we spent some time showing him how to interpret the diagrams (he does read as well, which helps greatly) he was able to build things on his own with little or no assistance. He now creates his own projects without the manual.
The '300 projects' part is somewhat misleading, as so many of the projects are just variations on the same theme, but there is definitely enough to keep him busy for a long time to come. As he gets older, we hope that he will begin to understand more about the "how" of all these projects, but in the meantime, there is much fun to be had in making lights blink, alarms beep, and propellers fly!
The company lives up to its promise to replace broken/missing parts for free. We have had to obtain three replacement parts, and we just filled out a web form and they showed up in the mail a couple of weeks later, no hassles and no questions.