I have concluded that using a professional editor is a very good idea. That said, I also think it is important to keep your editing goals in mind, and make them explicit. I have determined that my goals are not literary, but commercial. Brilliant phrasing and eloquence and perfection in word selection are great if they happen to happen, but spending endless time aiming for them gets in the way. All I desire is Clear, Correct, Consistent, Concise, and Done. Brain-cycles spent doing more than that, for me, are wasted.
Clear: Is it easy for the reader to understand what I am saying?
Correct: Does it follow proper conventions, and did I say what I meant to say?
Consistent: Does the style and format stay true to itself, so the reader doesn’t get goofed up on things that are not really a part of the story?
Concise: Aim more for Hemingway than Rand, but don’t be obsessive about it, because–
Done: Without this, the rest is wasted. Don’t play ivory tower, think Larry the Cable Guy, and just git ‘er done.
Your choices might be different, because you might have different goals… just be clear what your goals are.