I've been using the new Macbook as my main work machine for over two months now. I'm coming from a 2011 13" Macbook Pro, and haven't used anything newer extensively. That may mean I have a lower reference point, but it may also be closer to what a typical user would experience when upgrading. Here are a few quick impressions, and my thoughts on who this machine is best for.
This is my first computer with a Retina screen, and as expected makes a big difference. I can't imagine not having a high definition screen on a new laptop, and for that reason I wouldn't recommend the Macbook Air to anyone unless price or battery life was the primary concern.
I primarily use my computer for email, creating documents, web browsing, and watching the occasional video - nothing too taxing. This machine feels much faster to me than my old Macbook Pro, even though that had a faster CPU.
I attribute most of the speed difference to the Macbook's solid state drive. Double the memory doesn't hurt either. Whatever it is, don't worry about this computer being underpowered for common office tasks and web browsing.
After trying this computer out at the Apple Store, the keyboard really worried me. The keys have a very small amount of travel, and it takes some getting used to. Also of concern, the keys are spaced differently than older Apple keyboards, so some adjustment time is needed when switching between the Macbook keyboard and my external Apple wireless keyboard.
I don't love the keyboard, but it's not a deal breaker for me. If you are picky about keyboards and plan to write a novel on this thing, you may want to look elsewhere.
This is more of an annoyance than a serious concern for me. It's a bit annoying that an $80 adapter is required just to plug in my iPhone or a USB drive, but I rarely do those things anymore. I do need to carry that adapter around for when I do though.
I bought this machine because I wanted something very portable. I made some sacrifices to save size and weight, and I'm happy with my decision. If portability isn't one of your top two or three concerns, this probably isn't the Macbook for you though.