Got my Injen CAI for my 3.5 via NISMO yesterday.
Well packaged, all parts accounted for, very nice build quality, excellent detailed instructions, de-install/install took me a shade under 40 minutes, next time (like when I go back to the dealer for another recall) it'll be more like 20, gotta be careful that first time.
It's much more responsive when you hit the gas! It's also getting better mileage:
1-1k 24.6 mpg
1-2k 26.1 mpg (more Hwy, no morning warm ups)
50 miles mixed driving since I put it in: 26.8 mpg
My estimate of HP gain is 12 +- 3
I was planning on waiting until my exhaust feel off, but, that may be hard, once a really fine system hits the market, I'll probably buy, with that I could actually get into 27-28 mpg bracket, and still be smoking Bimmer 330's, TT's and Boxter's...
It's quiet when you are just going along, a tad more deep rumble.
When you are doing medium accel it's noticable, but if you were listening to music at medium volume, all you'd get would be the vibration, not sound.
At high acceleration it's loud, my previous experience says that if the exhaust is modified that it'll actually quiet down a bit. It's OK, after a few weeks I'm sure I'll hardly notice.
w