I think that it would be helpful to know what you are expecting from NEXTSTEP, OPENSTEP and Rhapsody... Because sometimes people's expectations of these operating systems exceed what they'll actually find using them.
I made this table of applications I use in Rhapsody and their comparative availability in both OPENSTEP and Mac OS X, and maybe this would be helpful in seeing that most of what I use is already available in Mac OS X.
There are a few apps I use in OPENSTEP that aren't on this list, like FrameMaker 3.2, Geomview, Chronographer, the Lighthouse office apps and Reality 3D, but I use FrameMaker 6 in
Blue Box in Rhapsody 5.6 and Geomview on my SGI Indigo2 IMPACT, so I don't spend that much time in OPENSTEP these days anymore.
If you are planning on running any of these operating systems in emulation, I would be of little help there as I really haven't attempted anything like that in the last 5 years or so. I stick pretty much with real hardware for all my needs...
- Sun SPARCstation 10 (OPENSTEP 4.2)
- IBM ThinkPad 760ED (Rhapsody 5.1, OPENSTEP 4.2)
- PowerBook G3 Wallstreet (Rhapsody 5.6)
- Power Macintosh 8600/300 (Rhapsody 5.6)
But I think that anyone wanting to jump into these operating systems should be made aware that it is nearly impossible to get license strings for most of the apps I use these days. So a lot of the functionality that lets me work in them today wouldn't be available to someone just starting out.
As for compatibility of apps from one operating system to another, compatibility was broken from NEXTSTEP/OPENSTEP to Rhapsody and broken again from Rhapsody to Mac OS X... and there are apps that worked in early versions of Mac OS X that won't run in the current version today. So while these all may share a common foundation, they are still very different operating systems and application environments from each other.