There are already a couple of XR videos up on YouTube, but they are on a smaller channel that uses the alpha test rules to show how well the Rampant system works for sci-fi. Spoiler alert: even the alpha test rules work really well, and the host plans on doing more videos on XR once the book arrives in my mailbox on December 1.
I've never played Grimdark Future. It would take me some time to put a well-informed comparison together.