I love what you guys have done with the approval mechanism. I think this is much better than the way that Github works because it lets you say that you approved the pull request but still leave it open for other teammates to look at.
Here's how we've been using this: If David submits a pull request and I'm the first person to approve it, I do NOT merge it. I leave it open for others to look at. We don't have a strict policy on who reviews the pull requests and when. Sometimes, a day will go by and no one else will review the pull request and we deem it okay for the owner of the PR to go ahead and do the merge. (All of our users have read/write to the repos).
It would be cool if you could enforce that it had to be reviewed before it could be pulled in by the owner.