From what I've read the double idler ones can cause problems. The 2 idler gears are floating between the cam and the crank. As the power is put through them to drive the cam there is a fair amount of thrust upwards on the cam that can cause cam bearing failure. The gears also move around in relation to each other so they don't mesh very well and that's why you get so much whine from them.
A good single idler setup where the idler is fixed to the cam cover or the block is a far superior setup. The gears stay correctly meshed and the thrust load on the cam is reduced. They also run quiet.
I've been planning on fitting this setup to my motor.
Cheers