I used to use a mail drop in Texas for My Very Own Mail. It was very convenient, in that it was right across the street from my house, and there was never a line. The post office always had this God-awful line that took forever!
I did pay for that convenience, let me tell you! I always had less than 10 subscribers back then, so it wasn't so bad, but, now I definitely wouldn't use them. I have, usually 18-26 packages to send out every month. And that convenience charge would definitely add up!
But another great feature of the mail drops is the mailboxes - if you're planning on using a POBox and the maildrop is closer than the post office, that would definitely be a plus.
So I looked up my city on this great mail drop directory, and we don't even have one. So I guess I'll continue using the post office, even when I get my POBox.