I only partly agree. A writer's group is a great starting point, but once you've got the basics down you may need more help. The feedback I got from agents was wildly different from the feedback I got from other writers. Writers tried to help me patch holes and rough spots while agents listed what they saw as major issues and weren't afraid to tell me to rewrite heavily. I would assume than an editor would have the same kind of ruthless eye as those agents for what needs fixing and why.
Well, I did say "a good writers group." A good writers group will be as ruthless as necessary; a bad agent or editor will tell you their opinion without holding back, but it may be a bad opinion. (Just because they're professionals, doesn't mean they're perfect.)
I've been on both sides of this issue, as a writer and a story analyst (the movie equivalent of an editor). So I've taken classes, been a member of writers groups, and dealt with agents and editors.
The pros of using an editor: A one on one relationship. No waiting. Someone with cred.
The cons: You have no idea how good a particular person truly is. You have to pay to find out, and even if you like their advice, there's no guarantee you'll be published. Considering editorial rates, that's a process that becomes very expensive, very quickly.
Granted, you have to exercise good judgment in finding the right writers group (a bad one is useless) and in taking the feedback you receive from the group. But the same would be true of an editor - and you're not paying the group.
Finally, committing to a group helps to ground you in the fact that writing is a long-term commitment of time and energy. A marathon, not a sprint (or a lottery).
