I rarely make a definite decision on whether or not I'm going to read a book by it's first line. That being said, when I do read a book that has an awesome introduction, I often give a little 'hmph', shift my position, look away from the book for a second, and then dive in. When I'm actually writing, I always take a ridiculously long time deciding on the first sentence and it usually ends up being something really simple, like 'It was raining again."