I know 3 guys running the TNT kit, 2 C5's and one C4. All of them love the kit. The TNT kit comes with a "power ring" for the nozzle installation. It's a very cool looking set-up.
I have a NX kit that will be going in soon. Whatever you decide to go with, I would recommend a wet kit, which the TNT and NX kits are, for the added safety of additional fuel being injected with the nitrous. Also, I would also recommend a fuel pressure safety switch. It will kill your nitrous if your fuel pressure drops to an unsafe level.
The installation is easy. The hardest part is running the wires and making all the connections. I'm building a wiring harness to make installation and removal easy. Plus, the installation has to look very clean. I hate sloppy jobs.
I would give yourself a weekend, depending on how many haors a day you will work on it. I would also HIGHLY recommend you draw out a wiring diagram BEFORE you start. Make a diagram of the complete system!!! This way, you can review everything and make sure it will work. I just did mine tonight at work (shhh, don't tell my boss).
One last thing to keep in mind, TNT is the ONLY manufacturer that gives their HP ratings for HP at the rear wheels. NX and NOS are crank HP ratings. My buddies C4 has a 125 shot and when we dyno'd it, it came out a bit higher then 125 HP, 134HP if I remember right (401 RWHP total).