I'd like to enter this fray by adding my two cents. I have hired an editor, Annetta Ribken, to look over the first part of TLOTV (Ch 1-30), and it has been, I believe, well worth it. It has not only helped with the chapters I have already written, but also with chapters I have written since. I am learning from her and my mistakes.
Anyone who would like to judge for themselves and has followed TLOTV can see the difference there. Chapters I have already edited have an E after the chapter number. If anyone would like to see how she edited one of my chapters I can email you with a copy showing how the comments look.
I cannot impress upon you the value of a second, preferably expert, pair of eyes looking through your work. Your writing will get better.
Here are the drawbacks:
1) Re-writing the chapters to meet the edits takes an awful lot of time. I have stopped writing TLOTV after the first part, and expect not to write any new chapters for a month while I tackle the changes.
2) You have to have tough skin and be willing to make the changes.
3) One edit does not lead to a final work. It will probably need to be re-edited a number of times before you have a finished product.
Anyone interested in using my editor can contact her at:
http://wordwebbing.com/ , and she can be followed on Twitter as @Netta50