The Smoothstepper works fine but it does lack backlash compensation. It has been promised since day one but have yet to be delivered, apart from that it works very well indeed.
As for the other controller you linked to - it's never going to work. First of all, the maximum step rate is only 29.5kHz and even if that is enough for you making it work properly with Mach3 will be impossible. External motion controllers for Mach3 works by having Mach3 calculate all the trajectories and then send them "in chunks" to the controller. The controller buffers them internally and "chews" them off one by one while Mach3 continues preparing and planning the next bunch of moves. The key here is that the controllers buffer can't ever "run dry", if that happens the motion stops until new motion data has arived.
This controller doesn't have an internal buffer so it would have to request an execute one move at a time, then Mach3 would have to constantly poll the controller to see if it's done with that command before sending the next - this would be painfully slow. Think of making a large circle, this would have to represented as an n-sided polygon (say 100 sides) the motion would go start-stop-start-stop-start-stop etc etc while working its way around the circle (polygon).
I'm sorry but it's simply not going to work.