This is a well thought out board with lots of nice features, like the optoisolators. It is very flexible and provides nice smooth motion. I had some problems zero'ing the gauge as it was not responding to the "!" command. The dev (boffin) responded to my questions which I find impressive seeing as I bought this almost 3 years ago.
I had to reflash the ATTiny to get the zero working and for me had to change the SwitecX25 library to get the zero working reliably. The motor would try to turn but seemingly not have enough power. It did move fine in normal positioning commands. This post https://github.com/clearwater/SwitecX25/issues/4 was the fix and suggested changing a delay constant used in the zero function.
#define RESET_STEP_MICROSEC 2000
I leave this here for any others with the same issue. May it save you many hours of googling about.
From presentation to documentation it's all there and nicely commented. I was up and running in under an hour. The boards were easy to build and the Arduino examples worked the first try.
<Edit 1/1/17> I have to point out how amazing this product is. It allows you to hook up to 26 X27 gauges to a single pin on a uC/uP. All the stepping and the step speeds are handled by the ATTiny on the serial gauge board freeing up your uC to something else or go to sleep. The maker has added an 'init point' mode that makes storing and restoring gauge position a breeze, super handy if you're turning off the gauge between updating to save power.
I bought a couple of these and they are really a fun item to play with.
I used one to "repair" an old steam pressure gauge which was mechanically beyond repair. I didn't need the full 315 degrees as there were mechanical stops which I chose not to remove. I used the original needle with some araldite and nobody will ever know it is electronic now.
Note: you may have to take smaller steps with heavy needles as the needle mass does affect the stepper performance (read the data sheet and look as the code for more information).
Something to watch out for - if you have a mechanical stop and you try to step beyond it, the display will no longer be correct as the stepper motor has no feedback. I thought that entering a position of 0 would execute a zeroing action but it doesn't. I am going to add a re-zero command so that in 24/7 use I can make sure the needle position is correct by issuing a re-zero from time to time. For now, power off and on and the dial will re-zero.