Arise from the grave...
I bought a 1.25GHz mac mini G4 about a week ago. Problems -- well, getting OS 9 set up on it was difficult, because Disk Utility doesn't always present you with the option to Install Mac OS 9 Drivers. I had some POS 1080p monitor that ran fine with a DVI -> HDMI cable under OS X, but not under OS 9. No resolution was compatible under OS 9, so I had to use DVI -> VGA. However I acquired a 1440p 144Hz monitor recently and that works just fine under both system versions, no issues. The only thing I don't like about the 1440p monitor is that it is more difficult to swap between display inputs (i.e. my main machine runs on DisplayPort, the mini uses one of the HDMI ports, but it's a OSD menu dance to toggle between them).
As others have mentioned, yes Bluetooth and WiFi won't work. A number of people over at MacOS9Lives have indicated issues with 1.5GHz mac minis. For some reason mine doesn't even have AirPort installed -- I was told they come with it by default. You have to load up OS 9 from the Option boot menu, as OS X won't detect Mac OS 9. When you reboot, you'll end up in OS X, unless you make some NVRAM changes. Sound -- it's fixed volume, you'll need a mixer or some kind of volume booster if it's too low or high for you unless you use a USB audio device. I also noticed my Corsair K55 RGB keyboard, which normally worked fine for pulling up OpenFirmware on my iMac G4, doesn't work to pull up the Option boot menu on the mini.
At the end of the day these minor issues aren't too big of a deal: I rarely use OS 9, and when I do it's for old stuff that works best natively, such as AppleTalk stuff. The bigger deal was to save space on my desk, as 1440p monitors (mine's a 27"/ 69cm) can take up a lot of space.
A bigger issue for y'all to worry about is the price of these things. I found 1 of 2 on eBay for $100, there were a handful pushing $300-400, which is ridiculous. They're not worth that much: they're so far behind in modern utility (as the browsers aren't compatible, you can forget about YouTube and stuff) and performance there's very little use case for them. The same can be said of the early Core Duo mac minis -- their GPUs are rubbish.