I've used several before as I used to sell higher-end grills (Viking, Wolf, Alfresco, etc.) and they are what they are.
At best they can allow you to produce a darn-near steakhouse quality steak. At worst you have a 1200 degree afterburner eating up a large chunk of grill-top real estate.
Disclaimer: I have been out of the sales game for a few years, and technological advances may have made them more versatile.
Having said all of this, I would love to have one on my next gasser
If you don't believe me, get yourself a thick ribeye, slather it with butter, salt, and pepper, and grill that bad boy a couple minutes on each side over a hot infrared burner. I could sell that puppy for $30.00 at any fine steakhouse!