I love my 15" rMBP but it is way too heavy for backpack commuting. I carry it in my backpack with other things around Japan, Korea and China for work. It's doable on its own but when you have other things like chargers, water, snacks, books etc the backpack can get crazy heavy. I end up holding my backpack from the handle instead of carrying it on my shoulders. Really not good on the shoulders. I love the new light 12" MB. Crazy light and the other advantage is it's USB-C and there are plenty of new cool USB-C powerpacks on Amazon so I don't always need to find wallworts. Of course there's no comparison for performance and I know 12" MB GPU and processor are underpowered but this would not be my main machine. It's just for quick on-the-go iOS, Android, and web development. Also if we use code injection in Xcode 8 (yes it still exists) we actually don't need fast processors for many builds/quick tests. I'm not worried about small screen real-estate either. I would normally buy one and use it for a week to try it out but I'm in Japan and they don't allow that here.