I use a biscuit joiner for one thing, and one thing only: Cutting slots for the L-shaped wooden “buttons” I use to attach tops to tables (and tops to table-like things such as my kitchen island shown above) and case pieces.
The buttons themselves are cut at the table saw, usually out of whatever offcuts I have scattered about, and are about 1-1/4″ x 1″ wide x 2-1/2″ long. (Actually, I cut them with an L-shaped tongue on both ends of a 5″-long (or so) piece, then cut each piece in half. Double your fun.)
Then, I set a biscuit joiner for the depth of the top of the tongue, mark the location and plunge. And if my tongue is a little thicker than the resulting slot, I simply lower the blade a bit and plunge again.
While I try to get the mark on the tool aligned exactly the same for the first and second plunge, if I’m off a hair (as I am in the picture below), I haven’t found that to cause any trouble.
The buttons are attached to the bottom of the tabletop through countersunk clearance holes, and located so that the tongue can swing into the curved slot(s) and hold the top in place. Because the tongue is held in place by the slot, there’s room for things to move as the seasons change.
It’s quick and easy, and almost foolproof.
There are, of course, more uses for a biscuit joiner; I’ve simply gravitated toward other techniques to tackle any given biscuit-appropriate situations.
But it is arguably a little silly to have a $100+ tool dedicated to this one operation – an operation that for most isn’t regularly called for. So here’s a link to an article from a “vintage” issue of Popular Woodworking Magazine. In “A New Manual for Biscuit Joiners,” you’ll discover 27 techniques to not only help you use the tool properly, but to make good use of it for more than simply cutting button slots.
p.s. Click on the images to enlarge them, if you wish. And below are some pictures from the underside of my coffee table to perhaps clarify things; the slots are in the aprons, the buttons are attached to the underside of the top.