Your question is very close to the one I answered here, so I can point you at that for some cobbled-together stats from soon after the audits were put into place.
Since then, we've had a lot more time to observe the review queues and how audits have impacted them. In my opinion, the audits have proven to be absolutely vital in maintaining quality within the review queues.
Almost every reviewer who tries to game the system for a badge hits the audits and either stops reviewing or reforms. When the audits were first put into place on the suggested edit review queue, it was stunning to watch reviewers who hit "approve" on everything get banned and come back as good reviewers. This pattern carried over to the other review queues.
We've seen what happens when you have a review system without audits. We're still finding spam that was not just approved but upvoted multiple times during the month or so between badges being added and the audits coming on line. I can almost guarantee that the site would go right back to that if the audits were removed.
There are only a small handful of people who keep trying to abuse the review system after being temporarily banned due to audit failures. Moderators have tools to suss out and deal with these bad reviewers manually, but there are only a relatively small number of them out there.
Sure, some aspects of the audits can be improved, and it would be nice if the system could catch a few more of those that the audits don't. However, I'm actually surprised that this technological solution to a social problem has worked as well as it has.
