You can see exactly what they're working on at any given time, and what the bugs are (save for stuff that's way upstream), what the devs are thinking, what delays there might be & why, and anyone can jump in and participate, offer fixes or ask questions. It's all done on an open mailing list. You can also test any of the builds (at your own risk), which tend to be spat out every few days (or at least once a week). Granted you can't see the development process for OEMs' work, though you frequently can test their internal builds which are often pushed to the main repos. They're as transparent as it gets.