I had the kit going on my relatively stock 302 last year. It just re-works the air intake so that when you close the hood, the vent closes ontop of the air box and the air coming through the ducts on the hood go down into the airbox making it functional.
I paid a total of $618 shipped to my door for the hood, and the functional part of the kit. If you buy both, they install the ductwork under the hood for ya so it's nice. IMO it was well worth it, check my pics in my gallery. You have a few options...
1. Just buy the hood and not the functional part...makes it cheaper
2. Get the functional part and use it with the K&N and just let the hood kinda throw cooler air near the filter (it's what I'm doing right now, I had to change my airbox setup when I went to MAF, but it's temporary)
3. Put the cervinis airbox on and make it 100% functional...(mine was like this for a year before I put the MAF on and had to change it)
If you make it 100% functional, it will increase power a smidge (def not the 25 hp @ 40mph they say thou). It's not really a performance modification, it'll change the sound of the motor more than anything. Know how the K&N kinda changes the pitch? Well, the ram air will make the motor sound more grunty and throaty when you punch it. I felt that it made my 302 sound 10 times better, and knowing me I probably would have spent $600 just on that nevermind the hood lol....but not everyone is like me. I enjoy the style of it and the sound change it made, performance is not really a reason to buy it.
Is it worth it? That's up to you. More people buy that hood for the way it looks, then they do for actual power gained. Just about any ram air system will add extra ponies. I've heard claims of between 5 and 15 rwhp, but that's second hand info. The way I see it, if I could afford it, I would buy it.