Hook the reader from the first page, first paragraph, first line.
Catch an agent's attention from the get-go--they don't have time to read until the action pics up after chapter five.
Everyone knows you have to SMASH your beginning. You should start out with a scene that evokes emotion, shows not tells the important traits of your main characters, and sets up the conflict for the rest of your book. But you also have to start with a scene that is true to the rest of your story. You should clue the reader in as to what kind of a story they're going to be reading.
Personally, I like dark romances with super-protective alpha males who aren't afraid to fight for what they want and paranormal creatures who feel real...but I still have to be able to sleep at night without thinking I'm being stalked by something that could actually be lurking in the shadows behind my door. Know what I mean? (I want to be swept away to another world when I bend back the binding, not living in my own with creepy creatures.) If I open a book in a bookstore and skim the first few pages yet read nothing about a vampire, shapeshifter, or some kick-ass magic wielded by some kick-ass hero or heroine, I'm going to pick up another book and try again.
Yesterday someone asked me, "How did you know where to start your book?"
I said, "Start from the moment your hero or heroine's life changes. Throw in some major conflict, possibly introduce them (and me as the reader) to the other leads in the story. And let the reader know what style of story (and style of writing) they can expect from the rest of your book."
The person said, "I have this idea...(then went into the idea which I'm not about to share here *wink), and the beginning is rock solid. After that, though, they have to go on this journey (think Lord of the Rings without the focus), and then the action picks up again later...about a third of the way through."
Let me say, firstly, that I'm one opinion...and I'm not even published yet so I don't know how much my opinion counts anyway. Secondly, I think their idea was good. They could easily start at the third point in the book, when the characters are flush into the action, and weave backstory in as they go. Their book wouldn't feel like a let down. (Meaning: If I started a book with a kick-ass action scene teeming with conflict, that's what I'd expect all the way through. If I came to a hundred page hiatus with nothing but a single character's journey, I'd probably put down the book and push it aside.)
So YES beginnings have to be good. YES, beginnings are important. YES, you should spend time crafting your beginning so that you can catch the attention of that coveted agent, then that coveted editor.
But this is true for the rest of your story. You should put just as much time into the last two-thirds of your book as you did making the first-third shine.
Now I have to put my own words to use. I'm working on the last 20K of Immortal, Beloved right now. The climax is approaching. All my characters are joining together to fight the impending evil. Oh, I'm excited. Someone's going to try to unleash creatures from hell...and he's going to succeed to a certain degree...but my characters will put up one hell of a fight. Some will die. Some will live. All will love.
Today, put your beginning aside. Stop mashing it up. Take a look at your middle and grow from there.