I've never personally used any, but if you look at the numbers they get incredible flow from modest port sizes. Lots of air, lots of velocity, yummy recipe to me.
I think their two main downsides are 1) the requirement to machine them for higher lift and 2) the "special" intake bolt angle that doesn't allow you to use old style intakes. Not a big deal if you're going to buy one anyway, but they're not as common at the swap meets.
I've heard of guys hogging out the bolt holes on an old intake so they could bolt it on, but I can't imagine that being ideal or long-term