Aftermarket intakes

I think that it depends a lot on the bike. I put the rad valve on my kdx and noticed a small improvement, but for the money involved, I believe that it would have been better (in THIS case) to just go with the reeds. I was able to compare my bike to a friends kdx (same mods except reeds only, not rad valve) and I was unable to actually "feel" much, if any difference. I would, however, like to try the moto-tasarini delta valve. In theory, it has more reed area and should allow for better response. I havent been able to find one for my 500 though, and my throttle response really doesnt need any improvement at this point.

I have used the Boyesen RAD Valve on my '94 KX250 and am now using it on the '99 model. It does make a noticable difference especially on the low end power. The bike has a better throttle response and a faster transition to the midrange. Those reeds are durable too.

Me and my friend bought our brand new '99 KX250 at the same time. After breaking-in both engines, I installed the RAD Valve on my bike. We were also running on similar fuel, oil, had identical tires, gearing, air filters, plugs,... and my bike always showed a better pull off the bottom.

However, wide open in 5th gear, both bikes showed exactly similar top speed.
I have not tried other aftermarket intake systems though.

I put a Delta V-Force 2 on my 2000 yz 125 and it seemed to help on low end a bit and some on top.:)

Depends on what bike you are going to put it in. I have a RAD valve in my KDX but have never ridden my bike stock without the RAD valve, so I cant really comment. I have heard it does next to nothing on a KDX but works well in other bikes.