Well, it's happened again. A writer I know went online today to complain that she'd paid an editor to review her manuscript and when she received the finished product, the errors and typos were out of control. In this case, the edited version was now littered with improper punctuation, incorrect dialogue tags, and spelling changes that reflected a European style while this book was intended for an American audience.
I really can't stress enough how important it is to know what you're getting before you pay an editor to work on your story. A true professional will not be insulted if you ask about punctuation placement, his/her stance on the Oxford comma (I'm a big believer, by the way), or if (s)he knows the difference between "affect" and "effect."
Ask questions, my friends. Ask for references--not just the names of authors (where you might just get the editor's buddies who'll tell you what a fabulous product they received) but titles of books you can review for yourself. Ask more questions. Ask for a contract. Ask more questions. Be absolutely sure you and your prospective editor will see eye to eye on the majority of your concerns before sending your money.
Publishing is a business as well as an art. Put the left side of your brain in gear before hiring anyone to do work that will affect your bottom line!