I have a Nest (latest, a 3rd gen? Purchased end of 2016 or early 2017). I love it, but would say:
1) It sometimes reverts a manual adjustment you make. So if you think ''I'd like it to be 2 degrees warmer now, as I'm cold'', it will do that for an hour or so, then it will decide that ''you always want it at 72 at 5pm, so I'll turn it back down from the manually set 74''. This is frustrating, as my wife and I would assume the other one had kept adjusting it.
2) Being able to log in on the way home from a trip, and turn the AC or heating on and have the home nice when we get home is great. In summer, with no AC, the house will hit 87F, so I turn it off when we're away a week, and turn it on first thing in the morning on our way home. Brilliant.
3) Yes, defaulting to ''away'' when we're not home is great.
If I could add a feature, I wish it could control my gas hot water heater too. I'd love to be able to turn it to vacation from my phone, and then on the morning before we come home I turn it back on.
Oh, it doesn't sense my dog. So if I'm out, and my dog is at home alone, it will think nobody is home. When you're out, it doesn't turn it off, but has some limits for max and min temp that I have set, so the dog won't freeze/cook, but will get a little warm
Oh, and it looks beautiful, and has many face options, and they turn on when it senses you walking past.