I used Delta curbside check-in at MCO in August. I don't recall there being a fee. I did give them a tip, but that was my choice.
Like PP, I found that there was no line at curbside check-in, and there was a line inside.
I'm a big fan of curbside check-in. Even when there are other customers out there, it feels like everyone is ready to go. If you have to wait at the ticket counter, as you do in some airports if you have checked bags, you will be waiting with people who have ticketing issues.