I got tired of waiting for AT&T and installed the insider app yesterday on my locked 640. I'm assuming it's locked anyway because I haven't unlocked it. I selected the insider release preview and it installed 10586.107. I've seen people talking about .218 being good but I'm not sure how I get that. Do I have to select the slow ring?
Otherwise, I have to say I really like it, it's way cooler than I expected. So far a few minor wrinkles but nothing I can't handle. It's slightly laggy at times but not bad. I wish the soft buttons would retract sometimes, and I don't know what happened to the Game center thing. All my games are just individual, ungrouped apps now, although I guess I can group them manually.
Edge is pretty quirky at times but pretty good for the most part. WPCentral is still awful of course, but that's true on any browser.
I also noticed that everything you run is still there in the background, apps don't self-terminate any more when you hit the back button. You have to hold down the back button and kill them off manually. Not sure if that's going to be permanent. So far I'm impressed overall though.