We recently published on Current Directions in Psychological Science a review of MTurk as a source of survey and experimental data. We discuss the traits of the MTurk samples, why people become MTurk workers and research participants, and how data quality on MTurk compares to that from other pools and depends on controllable and uncontrollable factors. The Psych Report published a nice summary of the paper, that you can find here.
Paolacci, G., Chandler, J. (2014). Inside the Turk: Understanding Mechanical Turk as a Participant Pool. Current Directions in Psychological Science, 23(3), 184-188.