Really nice! I only wish there was something similar in Python, Rust, or Go. I really don't like the Salt and Ansible approach of (ab)using YAML for command logic, which reminds me about how in the past people invented XML-based scripting languages... just because it was easy. Rex is cool, plus, more SysOps know Perl than any other language (except maybe Bash), so, this is a good selling point.
Although I use Chef on most of my servers, I like Rex because it doesn't require any client/agent running on the machine.