I had roughly the same experience as everyone else on this board. The group interview seems to be standard fair as things go. First a group introduction, followed by being broken down into a group to talk about the mission statement, followed by a mock sales pitch to a fellow interviewer. It was a fun experience, but I can't help thinking that it isn't really a good way to capture the strengths of the interviewees. I, for example, did fairly well until the final stage of the interview where we were meant to role play salesperson and customer within our group. At this point, it really is the luck of the draw with who you get paired with. If you get paired with the wrong person (say, someone who wants to show off as opposed to be cooperative-- my guy,) it will reflect badly both on them and you. There's very little one can do about it.
Other than that, it was a good experience. If I were corporate REI, I would try to make the interviews more focused, but I doubt that the fact that they'll lose a few good candidates because of the fickleness of their interview process is really going to make or break the company. It's a good company and I'm sure 90% of the times the method works.