My son is a huge fan of K'nex, but I am not so much. However, this Web Weaver Coaster was definitely one of the coolest options I've seen so far. I love that the track is glow in the dark, so they can see their handiwork in the dark. I also like that it's a self-propelled car, so it gives them a lot more options on how to build the track. They can do a horizontal or vertical design, and it will works ok.
I will preface this by saying that K'Nex are nothing like legos, and parents need to understand that K'nex pieces are harder to put together. You have to make sure everything is facing the right direction, you have to make absolutely sure you have the right piece, and if you make a mistake, going back to fix it can mean undoing a lot of work. My son is very patient and detailed, and has been into building toys for years. Other kids have not had the patience, so just be aware of that when buying this. This is not a toy they can put together quickly and start playing. It took my 10 year old about an hour to put it together on his own, but he's spent hours more "tweaking" the design to his liking.
Again, this is a fantastic toy if you have a patient child who likes to tinker.