The 2015 MacBook is thin and light; it’s the thinnest and lightest Mac. If you value those two features above all else, then it’s for you. If not, it’s not.
I value display quality and reliability. If my computer freezes or crashes it doesn’t matter how beautiful it is. I’m typing this on a Panasonic Toughbook CF-52, while the 2009 17-inch MacBook Pro sleeps right next to it. Since upgrading to OS X 10.10 Yosemite I’ve had to deal with all sorts of problems. I’ve had to find workarounds to prevent the sleek machine from freezing — coming out of sleep, and randomly — and from Finder windows graphically breaking up and not responding. (By the way, I completely disabled Folder Actions and that helped with freezing when the Mac wakes from sleep, but I haven’t had any luck fixing random freezes. The Finder does not like it when you view files in Column View and hide application icons. I unhid the icons and the problem went away.) I can’t work on a computer that freezes whenever it feels like it.
Surprisingly enough, my Panasonic Toughbook CF-52 running Windows 7 Professional 64-bit with Service Pack 1 has been more reliable. Not 100% though: the rugged machine did spit out a blue screen of death, but just once. I’m pointing the finger at Windows Live Mail 2012.
Reliability is important, and so is display quality. By quality I mean just a couple of practical things. On a laptop I don’t mind if viewing angles are not 178/178, but I’ve been coding a bit, so a matte display is a must. I can’t imagine having to stare at a shiny screen all day long. Actually I can, and it sucks. The second thing is aspect ratio. I didn’t buy my computers primarily to watch HD videos, I bought it for work and play. An aspect ratio of 16:9 is out; I find myself more productive with more vertical pixels so 16:10 is in. Both my MBP and Toughbook have a 1920×1200 pixel format. (There is one thing about the MacBook that is weird: the hardware-level pixel format is 2304×1440, but you can only run it as high as 1440×900. 1920×1200 or 1680×1050 would have been nice, but perhaps the user interface elements would have been too small.)
This is a long-winded way of saying the 2015 MacBook despite it’s sexiness, thinness, and lightness isn’t for me, primarily because of OS X 10.10’s unreliability and the glare on the display. There are of course other reasons such as the lack of ports, lack of CPU power, lack of GPU power, the expensive price, etc.