I've used them for about the past ten years exclusively, except for my recent buffalo hunt in Australia (see Dangerous Game forum). I wish I could have tried them on the buffalo, but Dr. Ashby's work indicates that Grizzlys are best for buffalo, so I used 160-grain Grizzly's on both bulls, with great effect.
That being said, I have killed many boar, bear, mule deer, moose, and a bunch of bull elk with Ribteks, and will hunt with nothing else as long as they are available. There is no stronger broadhead----period. Ribteks are one-piece, no welds to break), and the ribs seem to abrade the hide just enough to keep the hole open (as opposed to a narrow slit), making much-better-than-average blood trails.
I once shot a spinning elk, smashing the thickest part of the femur into seven pieces. The arrow continued all the way through the body and into the far shoulder. It was still sharp enough to shave with when I gingerly pulled it out of the chest cavity. Try that with your run-of-the-mill broadhead.
One neat thing about Ribteks that I learned while publishing Instinctive Archer Magazine is that the edges aren't ground away like other broadheads, they are stamped into shape, which means there is more steel there, it is just highly compressed. That means stronger. And man do they take a scary edge.