I just found out about Whizlabs a couple of days before my AWS Sysops test was scheduled. Unfortunately, I didn't pass the test but not due to the Whizlabs practice tests. I just didn't have enough time to go through more than two of the practice tests. I'm sure if I had gone through the tests a week or two sooner, I would have passed the test.
In my opinion, WhizLab's practice tests are the BEST AWS practice tests I've run across. Some of the questions on the two practice tests I finished were almost EXACTLY like the official AWS exam questions. The practice tests do a good job of covering the different topics you will run into on the AWS exam.