I usually read the samples online, such as the 'look inside' feature at Amazon. I'm only checking for writing style, such as use of adjectives and adverbs, rather than the plot. This is especially true for free, self-published works: I want to know the writer can write!
I have seen samples such as Mike has mentioned, where it's all front matter (intro, table of contents, preface etc) and nothing of the novel's prose. If the author is incompetent enough to let this pass as a sample, I won't download the book, even if it's £0.00.
Does anyone remember this site:
http://page99test.com/