What do you guys think about getting a K & N air filter for these cars?
Are they worth it???
Is it bad for the car?

Try that little "Search" button, above.

I did and found people asking that question on the newer cars (I have it in the rest of of my cars), but everything for this 380sl has been different so I though I'd ask.
I guess it would have helped to specify that I have an 81 380sl with 140k on it.

Well, you won't have a MAF to worry about fouling, but I still wouldn't run one for a street engine.

You're unlikely to see any real power gains. The added noise may make it appear to be more powerful, but without other engine modifications, the factory filter is sufficient for airflow, and far superior for filtration.

I've run K&N's on bikes, but those engines get torn down a lot more than I like to do on cars.

Your car, your call. That's all there is to it.