I have one and it works very well for thicker leather. I've made a lot of belts, knife cases, holsters, etc with it. I use mine mostly with #138 or larger thread and it works fine. If you are looking to use smaller thread, it probably isn't the right machine. I can also sew through thin Kydex for hybrid type holsters.
As far as getting it tuned, I haven't had much trouble. You need to make sure you're using the right size needle for the thread and type of needle for the material you're using. If you use small thread with a big needle, you will miss stitches, the thread will ride back up with the needle and not make a loop on the way up so that it gets hooked by the shuttle. You also have to make sure the needle is in straight and positioned so that the shuttle will pick up the thread in the scarf. You also need to make sure you have the tension adjusted right on the shuttle, then adjust the top thread so your stitch loops in the middle of your material, you want a lot of tension on both to make nice tight stitches.
It's not a fast machine by any means, but it works well if you treat it right and it's way faster than hand stitching. It's probably a better machine for someone who isn't comfortable with sewing machines in general. The Boss will put 1 stitch at a time exactly where you want it, like in a stitching groove. I've put thousands of stitches in with mine and haven't had to do anything other than the recommended lubrication and occasionally replace a needle.