This is a highly underrated feature.
The problem, of course, is that none of this works when the lights are off - like when you're in bed - which is one of the most common use cases.
Images of options in phone settings: http://imgur.com/a/lyq6q
They seriously do that? This alone could get me to switch. Switching from the iPad 2 to the iPad Air 2 was painful because they removed the physical switch that could be used to lock orientation. I used to use it for this. Now I have to swipe up (sometimes twice depending on the app) toggle rotation and then swipe down. First world problems. I know. But it's really annoying. To just detect the situation with no interaction is absolutely amazing. It's the sort of subtle solution I would expect from Apple specifically in contrast to Microsoft.
It works great except when it doesn't. Pretty frequently I'll lie down, use the phone for a couple of minutes, and then while holding the phone perfectly still it'll go "HEY THE PHONE IS IN LANDSCAPE I'D BETTER ROTATE" all of a sudden.
Which is to say, this kind of thing is great if it's 100% reliable, but if you only get to 95% it's a nuisance. More often than not I just manually lock it because I expect the Smart Rotation feature to flake out, and fixing it afterward is more work than locking at the start (rotate back to the right orientation according to gravity, lock the rotation, then rotate back to my viewing position).