You'll probably find most of the responses here to be anti-DRM.
Pros: MAY help prevent some casual sharing between family members and friends, if that's something you want to stop.
Cons: Any time a book gets locked down with DRM it has the potential to cause problems where the legitimate purchaser can't access what they paid for.
Note that while it may prevent some small scale sharing (such as sharing with a family member) it won't prevent the book from being posted on files sharing sites, IRC, torrents or the like. It's a matter of a few seconds for someone who wants to post the book to such places to remove the DRM after buying your book.