I can understand both reading habits. Reading a less-than-impressive book is a learning experience, too. If you're a writer, it helps you figure out what elements of a story work or don't work for you, in order to decide what you'll want to incorporate into your own writing. If you're not a writer, you can still figure out what things you like or dislike as a reader, which will help you in future reading selections as well as when you give book recommendations to a variety of readers.
Generally, I'll read the first chapter or two of a book to see if I'm interested in reading the rest of it. If I'm in a particularly unforgiving mood, or if I reach that oft-reached point of being overwhelmed by the sheer number of books I own that I have yet to read, I might read only the first page or two in order to sort the book into "keep" or "don't keep." I do this several times a year, and unfortunately, because I acquire soooo many books all the time, I also have to give many away, often without being able to read them first.
I realized the other day that no matter how little we understand about ourselves or our identities--for instance, no matter how many journal entries I write about the fact that, at almost 22 years old, I still don't think I know enough about myself, how I work, what I want, how my experiences have shaped me, etc.--no matter how little we think we know about ourselves, we are still a solid entity to people who know us. Everybody has their individual quirks or philosophical outlooks or manners of speech that will make others say, "Yup, that's him/her. That's something he/she would say."
So that's why I'm comfortable with not finishing a book. Along the way, I also ended up talking about why I think the first page of a book is so essential and representative. I'm curious now as to your policies when it comes to finishing a book. Do you make it a point to finish every book you start? At which point will you stop reading a book? And how important are a book's first couple of pages to you? A ready, set, aaand...answer!