NX has a life time solenoid warranty, but the Zex kit does make for a clean install. My brother and I have used NX for over 2 years, my cousin has used Zex for almost 5; no problems on either end.
If you can T off the fuel line just before it hits the rails instead of pulling from the schrader. It's been proven as a way to limit the fuel pressure drop across the rail from the solenoid opening.
Just type in "nitrous" and read every thread that comes up, not something you want to go into without alot of knowledge. (Also search at Modular depot.com, alot of good info there too)
(lower timing, colder plugs, CHECK THE A/F RATIO!)