There are writers who write strictly for themselves, who are too shy or nervous ever to show their work to anyone. Then there are writers like myself, who will sit atop perfect strangers and read our work aloud to them if we can get away with it. But when your looking for valuable feedback, to whom should you turn?
My short answer is: anyone and everyone.
The conventional wisdom seems to be that you should avoid family and friends. After all, these people know you. Some of them may even like you. Either way, their perspective is presumably too biased to offer honest feedback, or even to evaluate your writing objectively.
This same conventional wisdom encourages a writer to turn only to experienced professionals. Editors, literary agents, publishers, and the like. After all, these people work in the field, and surely their opinion will be far more expert, right?
As with a great deal of conventional wisdom, I disagree with adhering too strictly to this approach. The logic employed is solid as far as it goes, but it only covers a tiny part of the whole. Is it necessarily true that none of your family or friends can offer an unbiased, objective opinion regarding the quality of your work? If you have the talent and skill to become a competent, capable writer, there's an excellent chance that you have among your circle some intelligent people who love to read, some of whom even prefer to read really good writing. These people represent the audience you're really trying to reach: the discriminating reading public. If a friend of yours has read top-of-the-line, classic works in the genre you're writing, and they're sincerely impressed with your efforts, are their opinions automatically of lesser value because they know you personally? To a lesser extent, people who don't read a lot of what you're working on can also offer valuable feedback. If they generally don't like a certain genre but loved your story, kept turning the pages because they had to know what happened next, you've done your job as a storyteller.
The problem I've encountered with many professionals is that few of them seem to be looking for good writing and good stories. They tend to lean more to the marketing side of things. While reading your work, rarely are they exclusively focused on asking themselves “Do I like this?” “Is it well written?” “Is it holding my interest?” More often, they're scanning your pages and asking themselves “Can I sell this?” “Can I get a publisher interested?” “Is there are market for this right now?” These are legitimate business questions, of course, especially if one is looking to follow a trend as opposed to breaking or creating one.
You as a professional writer should be able to evaluate the various sources of the feedback you receive. How well-read is this person generally? How experienced is this person with the kind of stuff you're writing? Did they offer anything specific or just a general (and possibly insincere) “It was really good!” How much did they really get into the story? Did they specify any favorite parts or aspects?
Now I'm certainly not suggesting you seek out or only listen to exclusively positive feedback. Not my point at all. While writing my first novel, I received a number of comments from people about what was confusing, what seemed unrealistic, what seemed over-written, etc. I listened to all these comments and ended up incorporating a great many of them (because they made sense to me). I've always been a believer that highly specific criticisms reflect well on your work as a whole. After all, how could someone clearly and easily pinpoint what's not working unless it's standing out like a sore thumb amid a background of quality writing?
My point here is: get feedback from anyone and everyone you can, and pay more attention to the quality of that feedback than to the source. Like anything else, if you know what you're doing, you'll be able to know when positive or negative feedback has merit. If you don't, well, it's not going to matter what kind of feedback you get, let alone from where.