A nest isn't going to magically save you more money than any other t'stat really can. It just makes it really easy to adjust it and it senses when you are away. So, for people who never accurately programmed their old t'stats or never turned the temp down when they leave-yes, those people will likely see some energy savings. But, it isn't going to bring vast savings for a halfway accurately programed t'stat.
That said, I programmed my old t'stat, and I will tell you that my nest IS more accurate (program) because it is learning from my true behavior vs. what I think I should set it for. I know I am turning it down much more at night than I used to.
Also, how well it works in your setup is variable. If your heat has a very slow recovery time, then you are probably reluctant to make major temp swings.
A nest can't differentiate between you and your tenants. A body is a body. I believe you can incorporate them with other nests for multi zone systems-you will have to check with nest. They are not going to monitor other sources of heating or cooling like window units.
The nest is great-I love mine. Who knew a t'sat could be fun? But keep in mind it is a learning thermostat, not a programmable. Sure, your can "program" it via the web. But the point is you just turn the dial as needed, and it will learn and predict. This. you don't have to turn the heat up before you get home-it will quickly figure that out.
I attached screenshots showing how you can adjust the program (drag dots) and how it shows runtime/temps and if you or the nest made the changes.