Snap requires more horizontal resolution than is available when in portrait mode, so snapped windows disappear. They come back when you return to landscape, though.
Rotation is pretty fast, taking about a second to complete the switch, but with just enough delay (and a starting animation) to let you know that it's about to happen, so you don't get it by accident.
There is an auto-rotate lock. It's on the Screen Resolution control panel widget (reachable by right-clicking on the desktop, or doing a search on "orientation" or "resolution"), right where you'd expect it from previous Windows versions.
It's very easy to use the tablet in portrait mode for reading books or web pages, although the 768 width is a little lower than most pages are written for. Holding it is extremely comfortable, although be careful not to hit the Start button by accident. Folding the Touch Cover behind the tablet while in portrait mode works great; the Type Cover works too but it feels weird to feel the keys move under your fingers. However, the keys and touchpad on either cover are disabled when it's flipped behind the tablet.
The SIP (Software Input Panel) is absolutely usable. I'm coming to love the split-keyboard mode for short amounts of text; it's designed to be used by your thumbs while your hands hold the tablet at the corners. I almost never use the "normal" on-screen keyboard. The handwriting recognition really isn't usable without a stylus, as expected..
