I think the fact that nerf parts and guns that actually be created and function well makes for a definite advantage in customizability. Shit, you can probably make nerf parts out of play-doh. This allows for the gun to be truly a creation of your own and not off the shelf. Plus, it is possible to modify a great gun for a fraction of the price of a paintball or airsoft gun.
I bought a set of the tek pistols at kmart and they seem really promising. I opened one up to find a heavy-duty spring, a great seal on the plunger, and the whole thing was well lubed (ha...lubed). I am probably for now going to add a marker barrel to see how wistling micros work.
The drawback to this gun is that it seems to be made for small hands. The trigger is just a little to small for a comfortable grip.
At ten bucks, these are a good deal. Plus, they are pretty simple so it would be hard to screw them up.