As a writer, there are two things you need to understand about feedback. First, it is absolutely essential. You must heed it. Second, you must ignore most of it.
You can't be objective about your own work. Not really. And it's very difficult to see your work with a fresh eye. One problem is that you know all the characters, all the plot events. Everything makes sense to you. If you insert a character who hasn't been introduced yet, or get plot points out of order, it will look right to you. There will be problems that will be glaringly obvious to every reader on the planet EXCEPT you. You absolutely must show your work to beta readers.
And yet you must take what they say with a grain of salt. Every book, no matter how fantastic, has its detractors. I read timeless literary classics and find them unspeakably pretentious and boring. The literary types find my work trite or confusing. They don't get my jokes, and they aren't impressed by the action and suspense.
To some extent, the most relevant reviewers are your fans. People who like your work at least understand what you're trying to do. My fans will tell me that an action scene is confusing, or a joke fell flat, or a conversation felt stilted. This is stuff I can use.
On the other hand, you also need to have your work shredded from time to time by someone who doesn't like it at all. Trust me, the problems are there in your manuscript, waiting to be discovered. Your fans will be oblivious to them, or will overlook them. Your potential fans, though, people who might download a sample from Amazon and look it over, deciding whether to buy, those people will notice the flaws and be put off. A good critique group will contain people who dislike your work but are still willing to slog through it. More about that later.
Feedback from other writers can be precious. Writers see things regular readers don't. A reader might say that the book just didn't work for him. A writer will be able to tell you in concrete terms WHY it didn't work.
Be wary of writerly feedback, though. We writers are a compulsive, nit-picky lot, and sometimes we pounce on what seem to be violations of the rules we've so painfully learned. Passive voice! Adverbs! POV shift! I think we sometimes see problems that aren't there. We fixate on a lumpy spot on a tree trunk and don't see that it's perfectly appropriate to the beautiful forest around it.
Successful writing requires a schizophrenic mixture of hubris and humbleness. You need a certain amount of ego. You must believe that the words you write are going to be of interest to thousands of strangers. Yet at the same time you have to be open to feedback or you will never grow. Learning to pick out the useful feedback from the crap is a skill you may spend your life honing.
I have a small circle of friends I use as beta readers. They are reasonably gentle with their feedback, but they are willing to tell me when something sucks. If your friends are too nice, they're no use to you.
I get the rest of my feedback online. There are many, many online writers' groups out there. I like critters.org. The overall attitude is reasonably professional. No one is posting fan fiction or journal entries. The quality of the feedback is highly variable, but some of it is great, and the volume is high enough that you can find something worthwhile. Your mileage, of course, may vary. Find the group that's right for you.
There is another value to critique groups. Most of us only encounter reasonably good writing. Professional stuff that's been vetted by agents and editors, and polished up by an editor. You can learn a lot from good writing, but you can learn a lot from hacks, as well.
Every writer needs to see the mistakes the amateurs make. Once you've been through a few really bad manuscripts, or a few stories that are reasonably good but not really up to scratch, all sorts of things begin to come clear. You gain a whole new understanding of how a story can go wrong. Once you learn to spot the problems, you start seeing them in your own work. You can't fix them until you can find them.