During summer of 1995, I acquired an original NeXT Computer, a black Motorola 68030-based cube. I had the opportunity to upgrade to a 68040 about 6 months later, and jumped at the chance. Click here to see Byte Unix Benchmarks for Obsidian (the '040).
I've "hacked" the "backplane" in the cube to support both the 68030 and 68040 boards simultaneously. Now the trick is to either buy another SCSI hard drive (the easy way out) or figure out how to get the '030 to TFTP boot off of the '040 via ethernet. For more information on the backplane hack, see the Polylith Tech section.
I had used NeXTstep often in the past, but being able to play with it at home has really made me appreciate their user interface and operating system, which is based around the Mach microkernel. What's even more amazing than the user interface is that the Mach microkernel isn't a UNIX kernel, but its console maintains Posix compliance. Another nice thing about this operating system is that it now supports PC-compatible hardware, in addition to the original "black" hardware. Since the commercial version that users could buy through NeXT was expensive (and can now be hard to find), GNU has begun development on GNUstep, a free version of the user interface to run along with X.
Here are some of my favorite places to go to find out more about NeXTstep and GNUstep.