I'm personally going from the 2012 Retina Macbook to the 12" Macbook w/ Retina. I have to say, the only thing I am trying to get used to is the keyboard. I am so used to my "clicky" keys, that the feel is weird for me and I find myself pushing down too hard at times. Also, the touchpad is trippy because it's not actually moving, it's just vibrating and tricking me into thinking it's moving. Other than that, the last few hours I've had using this thing have been good. At first it went through some hiccups, but that's because I was sitting outside in close to 100F temperatures. I don't use my Macbook for anything overly intensive, so it's a nice upgrade for me. My rMB did me justice for these past 3 years, it was time to see her go to someone else. I love how these things hold value! I paid $1300 years ago and I'm getting a little over $800 for it now.
One thing I will add, I'm still trying to decide on the whole "one port" thing. I don't really plug in a lot of stuff, just occasionally my iPhone or my Apple Watch to charge them, but I do sometimes find myself plugging in an HDTV and not having an HDMI port might annoy me eventually. I guess I'll be seeing a dongle soon...