I received my seat covers from Christa last week and have now installed them. Let me give my assessment.
The covers are very well made out of a black synthetic upholstery cloth material with foam backing. They are definitely nicer than the canvas covers I've seen. They fit the GT seats well. They are held on by a half dozen elastic straps across the back and at the front corners. I had to pull and work the edge down around the seat belt anchor on the inner edge of the seat, but that's because there isn't much clearance there.
The covers are just that--covers. They are somewhat loose fitting, because to make them snug up to the original leather would require some way of attaching them into the seating area (not something you would want to try at home). They stay in place pretty well when I get in and out of the car, but they do slide around a bit, so I typically reposition myself to let them straighten out after I get into the car. However, they stay in place better than most seat covers I've used in other cars over the years.
I think they are very comfortable, especially given the hot humid weather we have been experiencing on the East Coast the past few days, because the cloth breathes much better than the leather. I also expect the cloth to be more comfortable in cold weather.
I asked Christa to put a pocket on the front of the covers, in the same place as the net pocket at the front edge of the seats. I generally keep my Valentine-1 there. If you want that pocket, be sure to let Christa know you want that option. She also offers an option for a pocket that hangs on the back of the passenger seat. That is about 10" X 14" so you can put a book, magazines or clipboard in there.
Bottom line: for $500 I think they are a great accessory and I recommend them.