After replacing my office chair, I finally bought the Lorell small glass chair mat. It is pretty thick! Prices have gone up since this thread was posted, but Overstock dot com sells the small one for $78.50. When I clicked through from Google search, they had some spin the wheel discount that came out to 15% off.
If you don't get that offer, you can still get 15% off by signing up for their emails. So $67 + tax for the 36"x48" mat, which isn't too bad considering inflation. Everywhere else is $90+.
Vitrazza is the "brand name" with the splashy TV ads and they have a useful setup video:
By comparison, Lorell is cheaply packaged IMO (but arrived safely) and didn't have much in the way of documentation. The included bumpers didn't adhere very well (so it's important to clean the surface before application). This mat isn't beveled like the expensive Vitrazza one, so it'd be a little hard to roll up onto the mat while seated. Unless I missed something, both sides of the glass are thus the same. Turns out my hardwood floor isn't perfectly flat, so one edge of the mat flexes a bit. I don't think this'll be an issue but obviously I'm not hopping up and down that edge if I can help it.
Lorell included 8 bumpers, so it's sensible to install 4 at the corners and 4 towards the center (see the video above). I looked on Amazon to see if I could buy a few more pre-cut bumpers, but I couldn't find any. It's some 3M adhesive-backed product; I suspect Lorell buys them by the sheet and has some machine that cuts out 1" circles. One of mine actually fell off and I didn't even notice it until the next day.
The main drawback is you don't really want to move this mat once it's in place. If you move out, chances are you'll have to figure out a responsible way of getting rid of a thick, heavy sheet of glass.