What sort of editing do you think you need? There are several different kinds of editorial input. In levels of decreasing pickiness and increasing abstraction:
1) Copyediting (typos, grammar, consistency, etc.)
2) Technique
3) Style and voice
4) Story structure
Allow me to shoot my mouth off about each of these:
1)
Copyediting. Unless I were rich, I'd be disinclined to hire a copyeditor. There's usually enough anal-retentive friends around to to get it into good-enough shape for submisssion. (Before I get flamed by the copyeditors, I'm not saying one's pals will do such a great job that it won't require professional copyediting after it is purchased--I'm just saying that they will do a good enough job with mistakes that it will be ready for an acquiring editor to look at it without flinching.)
2)
Technique. By this, I mean the basics of how you put a story together on the page, and all the writing-class cliches and things you can learn in books about writing. I don't feel it is appropriate to hire an editor to look at the basics of how you write and amend your tendencies. It's not a cost-effective way to learn. I think people are better off with crit groups, novel workshops, conferences, books on writing, Uncle Jim, and SYW forums (fora, really). Unless you are a celebrity and money is no object. But in that case, just hire a ghostwriter.
3)
Style and Voice. Not something an editor can fix. Indeed, I question whether good style and voice can be learned, since it is quite individual. I think it can be improved--by close reading, and by having people tell you when you suck--but why use an editor for this?
4)
Story structure. I think this is an area where a good, practicing editor could write a solid memo a few pages long, telling you why she would reject your book (pacing, a problematic character, believability, stakes) and where it needed to be shored up. But where do you find this gem of an editor? If you had a solid recommendation from someone (preferably a professional writer) you respect, then it might be worth it. But even then, keep in mind that editors don't all agree--I've had editors praise things in my manuscripts that other editors thought were drawbacks. You may get as much or more from beta readers.
The biggest problem I've heard about while nosing around the web is that the services of most editorial shops are self-perpetuating business deals, like the contractors who built the
Winchester Mystery House; that is, they encourage you to take on just one more service, just one more lesson; in effect, they turn into one-on-one writing classes that never end.
I actually think the diversity of novel workshops and community college classes and crit groups and forums like AW is inherently valuable. Readers--and editors--are diverse groups, and I think wide input is more valuable than employing a specialist.
On they other hand, if you're rich--do 'em all! (It's only money. And time.)