Depending on which brand you buy, you can actually get a corresponding "quick set" jig. Freud and Sommerfeld both have them. Its basically just a plastic gauge type tool that you align with the cutter. Once the tool slides into the cutter correctly, you are at the correct bit height. The tool adjusts for different thickness materials, and it works for much more than just the normal 45 degree lock miter bit.
If you go with another brand, you would just use the trial and error method (which actually isnt that bad....after a few tries I had my Whiteside lock miter almost perfectly dialed in for some HD-bought pine that I was playing with)
The joint it creates sure is nice. I read a how-to where someone suggested "burnishing" the corner of the joints, to create what looks like a seamless piece. I might give it a shot (I think they suggested just a stiff piece of plastic to round over each sharp miter edge). its possible, though, if the joint is tight enough that paint or the glue itself could just fill it nicely.