First off, DON'T DO IT! Writing in a vacuum is always a bad idea. As writers, many of us tend to be solitary folk, and we fantasize about the "romance" of sitting in a secluded cabin in the woods, toiling away on a typewriter until our masterpiece is ready to send to some editor who still manages to accept manuscripts via "the post."
Over the last year and a half, I've REALLY learned the value of having other people read and critique my writing. My critique group showed me many faults in my book, which helped me stay excited about the story. Mostly because their perspectives helped me improve the book far more than I could have on my own.
And recently, another friend of mine looked over my synopsis (despite a very busy schedule) and gave me more invaluable feedback. While some of her comments required more work on my synopsis, it'll only help my writing.
And on a side note, getting trusted writers to tell you what's "wrong" with your writing will make the criticism of professional reviewers sting less when (and that is WHEN not IF) a periodical publishes a less-than-glowing review of your book.