We have several of the Kid K'Nex sets, and I love them all. Even though he's now into Bionicle and Legos, my 7-year-old son still plays with them, and builds some remarkably complex machines and robots, all with suitably complex functions he is happy to demonstrate.
In the beginning, as a preschooler, he needed a little help building the featured creations, but I was amazed at how quickly he was able to follow the illustrations all by himself -- which gave me a little break and made him proud! Most of the different types of rods and connectors are color-coded, for example, green rods are longer than yellow, and red rods are flexible; and the illustrations are carefully angled and in color so that he could see every piece needed, and all he had to do was match the colors. It built confidence and gave him experience following directions before he could read. And, once he found that he didn't have to stick with just the illustrated creations, there was no stopping him!
The pieces are very sturdy, and, with the exception of the small blue rods, big enough that we had no worries about his younger siblings getting into them. They're all interchangeable with the other Kid K'Nex sets, and now that I see "girly" ones, I can't wait to get some for my daughter. (Previously, she's mostly wanted to make "flowers" with his set.)
Where Megablocks and Legos are great for structures, these are great for figures and wackier creations.