The Editorial Ass blog offers a good discussion of the pros and cons of hiring an outside editor to review your manuscript.
Enticing excerpt: “Remember that in most cases, you can't submit to the same editor (or, before that, agent) twice. That means that, again, you MUST be honest with yourself. If you are the kind of author who needs outside professional help, try to figure that out BEFORE your agent submits your manuscript to every editor in history. Many agents start out with small, exclusive submissions so that if the general editorial feeling is that the manuscript needs more work, the author and agent can go back and try again before using up all their submissions options. So in many cases, you'll have time to consider and reconsider.”