It all depends on how much you want to get your hands into it. I have one of the early enginer kits. I've been tinkering with it from day one to get it working right and working better. I hear the new kits are much better, but component quality is still an issue I'm sure as all the components are from China. Install is pretty easy. Mileage benefit varies widely. My best tank with the kit was 80 mpg (65 mpge) last summer. I normally average 55 mpg in the summer without the kit.
I only breifly got to ride around in Ben Nelson's new/used Prius with a hymotion kit. It seems much more capable of EV mode and assist power than my enginer kit does.
If you're looking for payback on any of these kits, I doubt you'll ever see it. It is a blast tootin around in EV mode though.
