There was a coach maker's rebate plane in the workshop tool box where I trained, so I gave it a go. I found it extremely difficult to use and apparently that was the general experience.
You're constantly wrestling with a couple of intractable problems. There's no cap iron so even at a high pitch there's a tendency to tear out, and because you're following a curve you're frequently planing against the grain. In order to keep tear out under control you need a super fine cut (and a very sharp iron), but that fine cut means its quite tricky to keep the iron in the cut. You just have to rotate the plane by a whisker and you drop out of the cut. Consequently I found myself making frustratingly slow progress with a series of short shavings. If you used the tool regularly I'm sure you'd eventually get reasonably adept, but it's a long slow learning curve!
Personally I'd use a scratch stock for relatively fine work like a musical instrument or most furniture scale components. For joinery scale work I'd be tempted to chop the curved rebate out with a chisel, might not be a tidy finish but I reckon it would take a tenth of the time and save a lot of hair rending and teeth gnashing!
