When I was using (and still using now) AmigaOS, I know I can trust it. It works with me, not against me. We seem to be on the same side. No hidden things going on, mucking up everything else. It just made sense, was streamlined and set out to perform the job in the best way possible. One cannot say that about many OS's.
Windows is remarkable in the way that the performance degrades with each new app you install. It just leads you one step closer to either reinstalling the OS or having to reformat the HD. The only way for the OS to continue working properly is to never install anything. Thank God the Amiga OS is not like that. Sure, it's possible for something to go wrong and you might need to reinstall AmigaOS or reformat the HD - I'm not saying it is completely free of this ever happening. But the design and structure of the OS do not contribute to this happening like it does with Windows.
I can only hope that the newer versions of AmigaOS have maintained that level of smoothness and co-operation in co-existing with other software. If 3.9 can deliver what 3.1 (and 2.04 before it) has, then I have no problem purchasing it.