I too use biscuits (and also the domino) for a number of shop and on-site tasks. Most if not all have been mentioned already. A key site specific task is providing strength (and alignment) to a mitered casing (as Paul shows with the Hartford clamp above). Another is attaching face frames to carcases (though I have switched to Domino for much of that). Last is adding trim to engineered material for shelves ... it excels for that job. I also use it for attaching boards of the same dimension. Like others I have found that Lamello makes the best biscuits (most reliably dimensioned for thickness) and I buy only theirs. A few years ago I replaced the cord on my PC557 with a Festool cord so that I could use it more readily with the integrated hose/cord.
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"the mechanic that would perfect his work must first sharpen his tools.” Confucius