I just finished a book that was the first book in part of a planned triology, and it ended with a cliff hanger. I'd gotten the book for free as a reviewer, but if I'd spent the $15, I would have been outraged that I wasted my money. As it is, if I see this particular author pop up on the publisher's site again, I won't touch his books again.
A good, well-written book with a beginning, middle, and ending and good characters gets people to buy the next book, not a cliff hanger ending.