I had two spare keys made at an actual locksmith here in town, so not a Lowe's or Home Depot. Think I payed 8 dollars for 2 keys. They had no problems copying them. I have heard some horror stories about using such keys on a daily basis though. Apparently the copied keys are generally of a softer type metal that will wear down in the ignition key and leave metal fillings behind that can gunk up the ignition and give problems.

I made the spare keys just in case I lose the original so that I can still get in and move the car, but I will get a copy from the Mercedes dealer right away if I ever lose the original.

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"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)

Unless someone has photos of an open cylinder with evidence of key shavings, I'd dismiss this as unfounded rumor. Mercedes locks, especially on the earlier cars, are nothing special.