I've been using OS4 since the developer's pre-release, and MOS for a little over a year. I'm rapidly beginning to like MOS a lot more.
I find it to be infinitely more stable and more compatible - 68K and WOS programs work great. I don't think I've tried any PUP stuff, though. I believe MUI to be a much better choice of GUI toolkit than Reaction since the level of control and customization over individual applications is far greater.
The label of MOS as an 'alternative' to AmigaOS is, frankly, misleading. As I've said it the past, if not for the name, you'd never know the difference. MOS basically _is_ the AmigaOS we've grown to love over the years. It has the same directory structure, runs the same programs, uses the same API, uses the same filesystems.
It's not an 'alternative' to AmigaOS, it's 'another' AmigaOS. If you've got a PPC card for your classic, give it a try. It's a little rough out-of-the-box, but stick with it and you'll probably come to like it.
