Sometimes, GIS people here in Puerto Rico ask me why I use Manifold? Well...
I have been an avid Manifold GIS user since grad school. I purchased the package for $145 in order to be able to do certain things that were difficult to do then using the ArcInfo version that was part of the university licence that I had access to. One of the things that struck me about Manifold (then version 4.5) was how irreverent its manual was. It was written with an almost literary style to it. It made bold claims and made sure call your attention to how well designed it was and how competitive is price was and how much better than anything available at any price it was. And it was! For vector data at least. It did not handle images or "rasters" at all. Being a fan, I volunteered to beta test for their 5.0 version which promised to be epic.
When 5.0 came out (in 2001 or so) I was surprised. Among the many improvements, one stood out. It used one file. And by one file I mean that all its components, including images were embedded in one file with a .map extension.
For anyone who has been working with GIS for as long as I have. This is a great conveneince. GIS file systems (still!) tend to be complex structures. And even on simple projects one has to deal with different formats from different vendors and files (such as shapefiles) have to be organized precisely and care must be kept not to ruin their naming structure or God forbid, delete them. In 2001 Manifold "solved" this issue. Import "everything" into one file. Which meant that one could carry that one file on a pen drive and move it from one computer to another without fear of loosing data of file mapping structures.
My next job using GIS was as Instructor at a local private university. I ordered both ESRI and Manifold (and Idrisi) licences for the lab and started to teach using ArcView. Students were confused not so much with the GIS language which so byzantine and cryptic (it needs its own dictionary) to some extent, but because they had to get used to shapefiles and geodatabases (personal and file types) and GeoTiff files and ECW files and MrSID files and xls and xlsx files, and CSV files, and all kinds of DEM rasters for elevation modeling. But I taught using ESRI software because it was and is still what "the industry uses" and for "resume building". But sometimes I would sneak in a short lecture using Manifold. And it was good. I could see students eyes light up in comprehension. I could see their happy faces of uncluttered understanding.
At one point I was surfing the web (preparing for class) and found this little gem by Art Lembo (now at Salisbury University). How do I do that in ArcGIS/Manifold: illustrating classic GIS tasks. This find told me that someone was also using Manifold for teaching. This little document became my lab manual. And I started to teach using Manifold partially. Aside from what I just mentioned, one of the benefits of using Manifold to teach was that students (and teacher) could focus on actual GIS tasks. My typical basic GIS course using ArcView ran based on the basic ArcGIS tutorial routine. Kind of like this, but using local data for Puerto Rico. So I ended up asking the students to just buy one of the ESRI tutorials for $50 bucks and to do it at home or school lab during the semester and hand in some intermediate products as a way to grade that they did that part of the class. It worked in that they got exposure to "the industry standard". In class it was almost all Manifold.
Manifold provides a more uncluttered environment to work albeit using a very different GUI than most GIS packages. This may be problematic if you already know ArcGIS, but given some effort, it is a rewarding experience. Some people think the GUI is unpolished, I think it is more terse to work with. It is more stable than any other GIS package I know. Has most of the analytical tools I use more often and a ton of features that I don't use at all but are available. It is very fast for most processes. It uses Spatial SQL which is awesome. For me at least, it is a fun package to work and play with. And it is inexpensive for a small shop like ours.