Amazon routinely provides the first three chapters of e-books for free. Other publishers and book sales sites usually offer similar arrangements. I've seen a lot of indie authors posting excerpts on their social networking feeds and blogs. There's an awful lot of free material out there for us all to sample.
Most people I've spoken to about this seem to have the opinion that reviews and reputation far outweigh the marketing effect of free chapters. How many people actually download the free chapters? How many then don't get around to reading them anyway?
Have you ever had a reader say that they read your sample chapters and then went on to buy the whole book?
Marketing our books is usually the hardest thing for indie authors. It would be helpful if self-publishing sites like Amazon gave authors some helpful stats on pageviews for our books and the number of sample chapter downloads. I would not be surprised if my monthly sample download count is substantially lower than my book sales.