Got to fiddle around with one today (My store's shipment came in so we had to pop one open to take a look). I gotta say, I'm pretty impressed.
The screen part was very lightweight but had a good, crisp picture and excellent sound (blows the Wii U's gamepad's speakers out of the water). Without the Joycons attached, it's basically got the weight, look, and sound of a higher end tablet.
The Joycons were a big surprise as well. You really can't fathom how small they actually are until you hold them. They're basically the size of the bottom half of a Wiimote. The connections are also very solid and will stay firmly in place once attached, but will also slide out easily once the release button is hit.
All in all, a good showing of solid Nintendo craftsmanship. I might have to get one sooner rather than later now.