When I started looking at the SmartRap Mini I found the project after it had been mostly abandoned by its creator in favor of the square aluminum box CoreXY approach. The design had an appeal to me, a minimalist appeal, as I like a minimalist approach to machinery. It usually leads to lower maintenance.
It was quickly obvious that a lot of history was in this project. I saw 3 distinct ways to drive the carriages; fishing line, herringbone rack and pinion, and GT2 belts. Other changes became apparent when studying the many forks of the project on Thingiverse. Different people had different wants from the design and often tweaked or changed what they wanted.
What I did not find on Thingiverse, the comments, the SmartRap official pages, YouTube, or the RepRap forums was a list of benefits and problems of the design. I DID find several lively discussions over fishing line vs GT2 belts. It appears that after GT2 belts became cheap, this became the preferred way to drive the carriages. Oddly, once tuned and running my SmartRap with fishing line produces far smoother results than my Anet A2 with GT2 belts.
Benefits:
Ultimately I choose to learn this design because it appeared to me to be very simple and straightforward. Fewer parts means lower cost.
The design is open to size, or technical changes. Many details could be changed, in this respect I think the original design DID meet the goal of being a good experimental platform for people learning 3D printing.
The print bed size is easily adjustable.
One could add a heated bed if desired.
Issues:
Fishing line will impose a speed limit, as will the hotend one chooses, and the power supply. Stepper motors are also important. There is a balance between dialing up the current, and skipping steps, between kevlar line and slipping steps, and software settings and skipping steps.
The cantilever beam system will definitely limit the size of the build plate that can be supported. The small Z carriage will begin to add droop as the X rods get longer. Fishing line on a 5mm shaft is limited to about 220mm for a build plate size. Beyond this and the line will roll off the shaft. GT2 belts can extend this size.
My design here will set a longer Y and Z axis by default.
Finding out how acceleration values can skew a print was, for me, about a 3 month learning process. I was convinced that the problem was in 'the bearings'. Finally, I figure the bearings had to be ok, what else affects this skewing of layers? Then I found acceleration and jerk values affect this. A few tweaks to Marlin and I had straight prints again. Fraying fishing line will also skew a print. Lesson learned.
Well maybe not maintenance, so much as tweeking tiny things to get 'better' prints. So far, once settled down, I will say the printer is being better behaved. But I have had periods where any change required lots of recalibration and tests to verify that things were operating well.