My first impressions:
It looks fantastic. Space grey looks great as expected (it's a little darker than the Space Grey iPhone 6, just by a bit). Silver is the one no one really talks about but I think it also looks amazing. I thought it would seem kind of ho hum, especially beside the Space Grey, but that wasn't true at all, I'd be perfectly happy with a Silver model. There was an rMBP next to it and it looked dated, when you take in the enclosure, trackpad, hinge and keyboard.
The keyboard is probabaly the most interesting part of it to me. When I first started typing on it I was instantly turned off and I was disappointed because I thought the keyboard was so bad. The travel is almost non existent, and while you know the key has been pressed and there's a subtle clacking, it's really jarring. BUT, after about 10 minutes of using it, I got used to it. It took my mind a little while to convince itself that the buttons were being pressed (it takes a very light tap for the keys to register) but after getting over that part of it I came to really like they keyboard. I made a fair few mistakes, something like 1 mistake every 10-20 words, but I could definitely see that improving with time. Right afterwards I typed on a rMBP, and the keys felt wobbly and the feedback wasn't as satisfying. If they add the butterfly mechanism and key size to the rMBP but maintain a similar amount of travel, the next redesign of the rMBP might have almost the perfect keyboard.
The trackpad is great as per usual. Force touch is trippy and awesome and totally convincing. There was one thing that might just be me personally. But I'm not a huuuugggeee fan of force click. It works, and it works well. There a couple things that bother me though. One, when I force click, I don't know quite how to explain this, but I can** feel that I'm interacting with a magnet. It feels like a click down, but it also feels like I'm slightly pushing against a force field or something. It's a little jarring, but again, that might just be me. Second, when you force click, releasing from the second level back to the normal level (non-click) isn't necessarily intuitive. Mostly because pretty much all buttons just have the one level. So sometimes I'd find I would force click, and release just one notch and move my finger, then when going to click something else I wouldn't realize that my finger was actually already in the first level and I have to let up on my finger even more. It's hard to explain, but long story short, it works as advertised, but it will take me time to get used to.
The display is incredible, I watched a 4k YouTube video and it looked great, no performance hiccups either. I liked the 1280x800 setting, everything is razor sharp. UI lag wasn't too noticeable in that setting to me, I resized Safari windows to test it and it was there, but it was also there with the rMBP next to me so I don't know if that's a Yosemite thing or what. I was using a 1.1 GHz model, think all the display models are the base config, and performance was fairly snappy. I never felt irritated or held back just exploring and opening apps. That being said I currently have a 6 year old windows laptop with a spinning hard drive, so I feel like any Mac would feel snappy to me due to the SSD alone. I'll be interested to see how much better the 1.3 GHz model performs to see if I would get it.
The size is great, it feels pretty insignificant in your hand, which is the best. The rMBP still looks fairly thin (especially because of the ports, don't know how much slimmer it could get with full size HDMI, USB, etc) but it has a heft to it. Not saying it's heavy(because it's not) but the weight is present. Not so with the MacBook.
Best designed notebook ever in my opinion. I have a serious decision to make when it comes to this vs the 13-inch Pro.