Yes it is better than an underhood filter. no better or worse than the stock airbox other than a conical air filter has more surface area.
By the way, if your current air filter is under the hood then it's not a CAI. It's an HAI. lol
Pulling air from outside if the engine bay is always desireable over pulling it from within.
The ram air effect of the march kit is negligable. Shortly after I installed mine, I was able to document an average of 8 additional miles from 13 gallons of gas over the Pensylvania Turnepike (no stop lights). In town, there was no measurable advantage.
I had one and I didnt like it, it looks ugly IMO and it didnt do anything. It came with my car, last year I sold it on ebay for 140 dollars. So idk some people really want it.
I have one and have had zero problems with it but then if my car goes 10 miles on an average week it's surprising. Does it do anything? There is a way to test it out and I may do it at some point. I would need to get a vacuum/pressure gauge and hook it anywhere in the intake tract. Then simply drive my car at speed with the scoop on and off and see if the vacuum shows a decrease. If it decreases with the ramair hooked up then the ramair works. If it stays the same then I wasted a bunch of money. I should be able to see a difference at 60 MPH in 5th gear.