also depends on the kind of book...
if it's a cookbook, i'll buy spiral over sewn any day... same for any other kind of technical-ish how-to... it helps to be able to leave it open and not have it close up on you, losing your place...
if you mean for submitting it, as jenn wondered, DON'T!... all mss must be loose...
if you're talking about a novel or chapbook or such, i agree that spiral is not the way to go...