A study by Peer, Samat, Brandimarte and Acquisti (2015) compared MTurk Workers responses to those from several other sites including CrowdFlower, Rapidworkers, MicroWorkers, and a University sponsored panel (CBDR). One interesting part of the study involves comparisions of results of attention check questions. About 86% of MTurk Workers passed two attention check questions in the study. This passing rate was significantly higher than the rates from other sites: about 53% of the CBDR panelists and 40% of the MicrWorkers panelists passed both checks, while only about 25% of the CrowdFlower and 6% of the RapidWorkers panelists passed both checks.
Yep, you read that right about RapidWorkers.