The first time I saw and used a NeXT machine was in Canada around 1994 - they seemed horribly cool even if they were starting to show their age. Merging cool hardware to a rock solid OS and a great GUI made NeXT the OS X of its time. In fact back when Apple was floundering around for something to replace the (Classic) MacOS - Apple bought NeXT (and Steve Jobs). Many thought BeOS was a better match for the Macintosh Way but while BeOS was a fantastic technology it was still immature whereas NeXT had been through several production iterations already (from NeXTStep v1.0 to 3.3 and then making the jump to OpenStep 4.0 to 4.2). OpenStep was an attempt to abstract the OS from the hardware and would run on top of Solaris, HPUX and on Intel x86 hardware as well as the older NeXT hardware which was eventually discontinued in 1993.

The NeXTStation 'slab' reached its zenith with the Color Turbo workstation - it featured a Motorola 68040 running at a whole 33MHz. The more distinctive NeXT Cube was also available in a Color Turbo configuration (although I suspect due to the increased price for the cube people stuck with the slab configuration).

In terms of performance the black hardware was a little underpowered - NeXTStep 3.3 or OpenStep 4.0 on Intel hardware ran rings around the old Motorola chip.

Some information about NeXT hardware is available at LowEndMac. Some OpenStep screenshots are available at GUIGuidebook.

Of course the most important thing of all is that the NeXT heritage continues to live on in Mac OS X.