I used Aviation Exam and found it sufficient. My main reason for going that route rather than Bristol was that I could purchase a CD for off-line testing, which synced with my online test history. Plus, it was less expensive.
I found the questions to be more or less accurate. Can't memorize all the answers anyway, and the tests are much easier when you understand the material. Although, it certainly helps to get a feel for how the questions are worded.
Aviation Exam has a poor explanation feature for the questions, while I think Bristol's was much better. However, they are attempting to improve this feature.