@Jeff
OS4.1 here ... I use my system every day. I don't think there is a "killer app" yet, but several people are working on software applications.
I started out with a Commodore A2000HD with AmigaOS 1.3. Later upgraded to AmigaOS 3.1. I did not have an accelerator card, so it was a stock 68000 CPU, running at 7.14 MHz clock speed. I added some more system RAM, a second SCSI hard drive, a SCSI CD-ROM drive and a MicroWay FlickerFixer card.
Since I was using the original 68000 CPU, I could not run software which required a 68020 or greater CPU.
When I purchased my OS4 system, I went straight from AmigaOS 3.1 with no bells and whistles to AmigaOS4, which incorporated many ideas from OS3.5 and OS3.9. So I had a very pleasant experience seeing themes and preferences that I had never seen before.
Over the past 4 and a half years, I have added 68K software which I could never have used on my old system without an accelerator card. Of course, I have also added PPC software as well.
Presently running OS4.1 with a mixture of PPC software and 68K software:
NetSurf, OWB 3.12, AWebPPC, IBrowse, WookieChat, SimpleMail, YAM, Final Writer 97, Personal Paint, TVPaint, Real3D, KingCON, WarpView, DvPlayer, AmigaAMP, AmiDVD, MakeCD.
Some of my old 68K programs (not listed above) require OS 3.1 or the classic Amiga chipset. I run the AmigaOS4PPC version of E-UAE to provide the right conditions for these old programs. E-UAE opens up an OS 3.1 Workbench ontop of my OS4 Workbench and then I am able to run those old programs (for example Shanghai game, Amiga Appetizer package and MicroFiche Filer).
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redfox