I've had a bit of time on the MacBook now. First of all I love the machine - it looks amazing and even better than you expect from the pictures. In Space Grey it's stunning and looks really different.
The keyboard is very fast and satisfying when you get used to it - though clicky and noisy. Trackpad is great, screen is amazing. Battery life for what I do (nothing extreme) hasn't been great, but never has been on any laptop for me. Plus it charges so quickly that you can basically do a half charge in your lunch hour, and also use USB batteries when they bring suitable ones out.
The speakers are absolutely stunning! No idea how they pulled that off, given how thin it is,
In terms of performance, I have the 1.1 model. It's okay - it manages to do most things. It's not the snappiest thing in the world, and there's relatively frequent pauses, stuttering in the UI etc. I am a little disappointed with that; but generally it's a relatively capable machine. I've tried multiple tabs in Safari, 4K video, light Photoshop work, a big photo library in Photos, light InDesign work. It's all possible, no huge lags or delays, just a general feeling of it not being all that smooth. But I think that's more UI/graphics based; rather than down to the processor. The 13" rMBP I've been using recently also stuttered quite a lot compared with my iMac, I think it's a limitation of the retina displays in general, and I'm hoping Apple will improve this across the board, before too long.
So, quite a capable machine. You wouldn't want to be going over the top for graphics with it, but as a secondary computer, I think most people will be happy, as long as they aren't expecting buttery smoothness in the UI (that we've come to expect from iOS and most Macs).
EDIT: Turning on 'reduce transparency' really improves the smoothness. Still not great, but much more usable. I'd say this is a necessity for this machine!