There is a Buy 1 Grand Mac Get 1 Free Med Fries & 1 Free Med Drink coupon in the McDonald's app, so I used that today to try it out finally.
This thing is pretty intimidating. An ordinary Big Mac doesn't phase me and actually feels a bit small, but this is an actual meal of a sandwich, so be ready.
It might not look that much larger in pictures, but it's around double size. You get a pretty big hit of the mac sauce in each bite due to the larger surface area. On the normal macs it sometimes doesn't always reach the edge due to how they prepare them.
I'd say try it if you like Big Macs, for sure. It still has that bready feeling the ordinary Mac does, and the patties, while larger, are still thin, overall I liked it for what it was. The mood might strike me to try it again sometime... but only if I'm famished.