We had ten boys here between the ages of four and eight and the Defiants were a hit with every one of them. We had straight floor races, trick races, curvy course races, and head to head action to see who could knock the others out of the way. The boys loved playing with the tall hill set, and also the flexibility of building their own courses and terrains. We must have gone through dozens of configurations during our party.
I love that although the Defiants 4x4s trucks are really cool, it still leaves most of the play to my boys' imaginations. They can be creative with the sets and their interactions between each other. The trucks are easy to use, easy to change batteries (AAAs which last about 3 hrs of serious play), and have held up well to hard play - even went we went camping and the Defiants ran through lots of dirt, rocks, and rough outdoor terrain. That's not to say we didn't lose the grills, headlights, and other embellishments on the body of the cars. Those fell off really fast, but my kids didn't care.
I think Defiants are a good bridge between plain vehicles and remote control vehicles. Plenty of room for on-their-knees-creative-fun, but can still run through the course by themselves. I would recommend these for boys (and any girls that like trucks) between ages three and ten. They are really fun for all ages. I admit to setting up a few tracks myself and running my Defiant through. These will keep us busy all summer!