I've seen a picture or two here where somebody has one. I don't think you need it it. I use a stone sometimes or just throw the pizza right on the grate without a stone. Looks more gimmicky then anything else, but that's just me.
Check out this post by and old friend Smokey Lew. He has a pretty fancy pizza stone you can get by with a regular pizza stone or even that piece of travertine tile BW referred to. Very similar post by Don Cash. For my setup I added two more fire bricks on the cooking surface and set the pizza stone on them, I think the theory is it is hotter in the top of the dome.
Don also posted a method of standing fire bricks on their side or your could fill soup cans with water than placing a pizza stone on top. The idea was to get the pizza higher into the dome where its hotter since heat rises. I have tried this a couple of times. Using this method I had my first successful pizza on my kettle.
__________________
Weber Blue Performer
Weber 22.5 OTG with Weber Rotisserie
18" WSM
Weber Jumbo Joe
UDS
Weber Q100
Santa Maria Style Grill
Not sure it applies to pizza by any means but noticed a few years back on brisket a inner lid directly over the top of the goodies makes some great burnt ends...crisps things up nicely..this be considering enough space around the bottom to let some hot air rising get up inside the inner lid. Sorta like a dropped ceiling thing. On my big pit an upside down Sams pan or a lid off a Weber kettle works well. Seems sorta like the same principle involved for the pizza gizmo.