That having been said, I'll try to cover a few of the finer points of Rhapsody 5.1 (aka Rhapsody Developer Release 2).
Images and Screenshots
If you are looking to find out what Rhapsody looked like this is the place... but not the page. My Rhapsody Resource Page and Apple Pictures Page have a number of shots of Rhapsody in action.
In December of 1996 Apple announced that it had acquired NeXT Software (formerly NeXT Computer). NeXT was best known for it's computers and innovative software (including the NEXTSTEP/OPENSTEP operating system, Enterprise Objects and WebObjects development tools). At the time OPENSTEP 4.1 was the current version of the NeXT operating system.
The plan was to try to bring the Mac look and feel to the underlying development platform of OPENSTEP. The result was a fifth version of what was now Apple's operating system. This new OS was given (yet again) a new name, Rhapsody. The first version (5.0) was made mainly from OPENSTEP and had the main goal of working out the bugs of a port to PowerPC. Because of it's close relation to OPENSTEP some apps worked on the Intel version of Rhapsody 5.0 without needing any rewriting at all.
The next version stood in stark contrast to the first in that it had a more polished GUI which felt very much like Mac OS 8 and that it had done away with many of the OPENSTEP structures which were still quite visible in the previous version (like the Next directories on the root level of the hard drive). More than these changes, the goal of this version of Rhapsody was not to work out bugs as much as to give developers a head start on making applications for the final release. Rhapsody 5.1 was the foundation of all Rhapsody releases to follow.
Historically, Rhapsody 5.1 is best known not as a developer tool, but as the last Apple OS (besides Darwin) to run on Intel based hardware. The reason for this is that Apple scrapped the Rhapsody 1.0 (aka Rhapsody 5.2) public release when the company started working on Carbon and started the Mac OS X project. Rhapsody Server 1.0 (aka Rhapsody 5.3) was renamed Mac OS X Server 1.0 to reflect the new direction. More notably, the Intel versions were dropped from the Rhapsody line completely.
Rhapsody 1.0 was used as the structural guide for Darwin. Apple took great pains in removing any proprietary software they didn't have control over and replacing it with open source versions. As Apple felt the look and feel of the Apple OS was one of there most important assets, the GUI (along with the Yellow Box application environment) was removed.
All versions of Rhapsody after 5.1 (5.3 - 5.6) would be restricted to Apple hardware.
My Experience with Rhapsody 5.1
I have spent more time using Rhapsody 5.1 than any other version of Rhapsody (or NEXTSTEP/OPENSTEP for that matter). I've installed it countless times in VPC, a few times on my DEC Celebris, but only once on my ThinkPad. I use my ThinkPad everyday.
The main question I get asked over and over is "what can you do with it?" I can actually do a lot. I have a web browser (OmniWeb 3.0) which is helpful as most of the resources I keep on my ThinkPad are in HTML form. I have a PDF viewer (OmniPDF and PDFView) which is helpful as that is the next most prevalent format I use. I have a word processor (I use TextEdit) and page layout, web design and illustration application (Create 5.1). And I have a spreadsheet app (Mesa 3) if I ever needed one. Given those, I'm pretty productive on my ThinkPad (much of the Rhapsody Resource Page was made on my ThinkPad).
This is not to say there aren't limitations. Rhapsody 5.1 is a 1998 operating system which had an almost non-existent user base. Further, at the time Macs were faster than PCs in most tasks and many developers stopped making both PowerPC and Intel versions of their software. When I started using Rhapsody 5.6 on my PowerBook G3 I realized just how large a discrepancy there was in the software selection area.
Still, I continue to use my little ThinkPad. When I moved from Rhapsody 5.6 to Mac OS X v10.2 on my PowerBook and upgraded it to a G4, it became the center of all my computing work. As such, it doesn't leave home too much any more. This has driven the ThinkPad back into full time use as my mobile system.