This is quite timely. I just set up OpenBSD on a new (to me) 15” PowerBook G4 and have been using it over the past week. Quite happy with how well it runs, and it’s astonishing that a 14 year old laptop can still be so good.
I also did this and was very impressed! It was my only laptop for about a week and I could pretty much do everything except run a modern browser.