I got this for my son (7) for Christmas, on a whim, because he had noticed it on a sweep of the K'nex aisle in Target about a year ago. I am over electronic games right now, and this was an extra "guilt" gift to mask the totally analogue Christmas. He built it yesterday, and it was a wonderful, challenging experience, and a huge success. It looks really cool when it's just running.
Cautions:
-The instructions are all visual. My son is a Lego veteran, and so is fairly confident about interpreting these images (which are like gibberish to me).
-When they say 7+, they really mean 7+. Otherwise, plan for a joint venture.
-The bendy track is somewhat fragile! I could see where the bendy track weakened from being disconnected roughly. It is soft plastic.
-Counting the chain links...ugh. I did that part.
Praises:
-It is a challenge that pays off. My son tried shortcuts when he thought he could, and used trouble shooting to figure out sticking points or flaws. There's definitely an "I did this!" factor to it, when it is up and running.
-Gravity is still kind of awesome, you have to admit.
-It's reusable! There are plans for additional configurations on the K'Nex website, and of course it can just go into the giant K'nex bin under the bed for free building.
-The process itself is valuable. After admiring the finished roller coaster, he moved right into "what happens if I...[run both cars together, move track, simulate crashes, run it backwards, tape figures/people onto the cars, etc]." Obviously a precursor to destruction, but lots of exploration along the way.
In short, it is an inexpensive kit, a fun way to spend time with your kid, and it provides a sense of accomplishment for the builder.