Ya know, if you can nail that first line in your book it is absolutely going to work in your favor. It's never not going to be a good thing. Charissa and James gave great examples in their posts from Sunday and Monday. Plus, "Best Of..." lists abound, and who doesn't love the free marketing of landing on those? Okay, okay, we also love the warm fuzzies of readers responding enthusiastically to our prose.
Is your work going to be an instant DNB/DNF if that first sentence doesn't grab the reader? I dare say, nah, as long as you've hooked the reader within the first page. My qualifier is that the longer it takes to hook the reader, the more readers you lose. Imagine the reader's attention being in a sieve, and the only way to plug the holes is with interesting content. At any point in the book, if you have too many holes exposed, the reader is going to get bored and put the book down.
That's not to say you should fall into the trap of obsessing over your opening line. I've seen too many baby writers feel defeated because they can't "entice within ten words." Courage, my friends! Slap some words on the page and keep going. Write the book. Come back and revise that opening hook (and paragraph) once you've drafted the story. By then, you know your character and your world, so crafting a sticky opening is easier.
Often, the first thing written is the last thing finished.
Am I a mistress of hooky first lines? Depends on one's taste, I reckon. Nonetheless, here are the first lines from the first books in their respective series:
From Larcout: Blood beings could be chattel or they could be char.
From The Burned Spy: The antidote burned worse than the toxin.
From Celestial Ascent (WiP): Summer’s night lay like a coarse wool blanket soaked in bull urine.
From Worthy (WiP): Negative humors held the color of wisteria glistening with fragile dew on a background of sinister blue.
Will those WiP openings change once I finish the drafts? Maybe. Possibly. Maybe not, though. I'm not done drafting the stories yet.