My bias is heavy, of course, but I'm happy to answer any questions. I used Rebol before Red, and these langs make it really hard to go back to others after you get used to them. They do suffer some of The Lisp Curse, where people roll a lot of their own stuff, and there's not a full ecosystem yet. If nothing else, they stretch your mind and let you think in new ways.

One thing that the old docs don't cover at all, because Rebol didn't have it, is the Red/System (R/S) dialect. It's a C level systems language. Red compiles to R/S, which then compiles to machine code (no external compiler needed), and R/S is, itself, a dialect of Red, so it takes the whole metacircular idea to another level. We describe Red as "A full stack language, from metal to meta". Other key aspects are that it's small, and you can build standalone programs with it. Easy to get started, as it's just a single EXE, no install required. Well, there are 3 now, because the text and GUI consoles and toolchain are separate.