I have run Air LIfts' kit on all 4 of my cars. I installed a new set the day I got my current GN.
As mentioned, they really help eliminate the body twisting when launching hard. Also, if you run a taller/wider rear tire with un-rolled fender trim, it keeps the trim from digging into the sidewalls on launch.
Couple of more tid bits about them.
If you remove the front sway bar for better weight transfer/weight reduction on the nose, run 10-15psi in both bags for street driving. This will effectively bring back the sway bar feel when taking corners. It won't feel exactly like stock, but the car won't feel like a cargo van about to tip over either.
The factory rear bumper has two perfect spots for mounting the air fill valves. If you remove the torx heat bolts that are used to bolt the bumpers vertical bumperets, you can put the fill valves there and hardly notice them when the black caps are installed. Over the years, I've had those valves in various spots until my best friend showed me that trick. And if you're a stickler for stealth, you can take a black touch up paint pen and paint the remaining portion of the valve, nut and washer to further hide them. IE: on a black bumper.
Route the hoses away from the tail pipes, or they can and will melt. I like to route them through away from anything that moves, then through the frame rails, then over to the fill valves. Zipties are your friend.
The two round plastic polymer pieces that come with the kit, that are supposed to be mounted under the bags, are not necessarily needed, as you can install the bags without them. But it doesn't hurt to install them.
It's best to squeeze the bags as much as possible, to remove as much air as possible. Then put the rubber caps back on the barbs to keep them deflated. Then push the bags through the widest coil spring spot and feed the bag up into the coil spring. Once it's in, pull the rubber cap off the barb and it will semi inflate to shape. Don't forget the squeeze clamps on all hose / barb connections. Those are a must if you intend to run high pressures and launch hard.
At the track, with bias ply 28x10.5 slicks or ET Street bias ply DOT tires, 12psi cold air pressure in the tires, I would run 25psi in passengers side bag and 7-10psi in drivers side bag. Launches of anywhere from 8-15psi works good and gives very little body flex. Given, also having all of the braces and lower body bushings dramatically helps as well.
In closing, if you plan on seriously racing the car, the air bag kits are a must IMO.
-Patrick-