Plan9 is to midwits what shit is to flies.
It wasn't that revolutionary back in the 90s and by now everything worth while was incorporated and improved by other OSes. I used a unix port of RC is a login shell from time to time and the networking functions in go that were taken straight from plan9 are a godscent compared to berkley sockets and that's about it.
9p is a garbage RPC binary protocol (so much for "everything is plain text") and half of rio is tardrangling some garbage about FIDs.
There's a reason why people like Drew Segfault love it so much, it's unix but le cool and enlightened. It's a treasure chest of retarded ideas that were rejected for a reason and now that people forgot about it you can larp as a computer connosseur rejecting modernity and embracing tradition (or the other way around depending on which angle you're aiming for).
It's useful to occasionally read up on it though, as with anything else.