@kiwa#MacOS9 on them is fantastic and on pair as the most highend as MacOS9 experience gets. And the amount of software for MacOS9 is huge and easy to get hold of via Macintoshgarden and MacintoshRepository.
Some of my posts in here is about the experience of MacOS9 on #MacMiniG4. I can recommend the journey of fiddling about with some nice old software and think about the 90's. I had some fantastic months of entertainment with it the last years.