Lots of people can write a good first page but to sustain it, that's my litmus test. If I flip to the middle of the book and there's a piece of dialogue that's just outstanding, or a description, then I'll flip back to the first page and start it.
I have this pet peeve which is when I read a novel I don't want to hit page 200 and have to go back to the beginning to figure out which character is which,