I originally purchased a 3M wifi thermostat only to find that it didn't work with my HVAC system as my HVAC system didn't have the extra power wire needed to run the wifi and the wifi won't run on battery power alone. Installing the power wire needed was going to be a hassle so I chalked this up a waste of time and money. It's just as well the 3M "app" (website) for controlling it remotely was a joke.
So I finally gave in and looked at the Nest. It's not cheap at around $240 USD. NOTE: The cheaper $220-ish model is the first generation model, so probably best to make sure you get the slightly more expensive 2nd generation model just so you have the latest and greatest.
I avoided the Nest for a while since it was one of the original Apple iPod designers that made this thing and I'm NOT an Apple fan. But I looked at the Nest website and it was very well made, has a little "wizard" to see if it will work with your system and that's what actually sold me on it. It worked with my 3 wire (air conditioning only) system without additional power wire.
So I bought one via Amazon (free shipping, why not) and it arrived days later. The packaging alone on this was very well thought out which is a nice touch, typical of Apple products, and reminded me of Dyson products. While packaging doesn't matter, if they put that much effort into the way it is packed that tells you the product was likely well thought out too. And it is!
Installation is simple, they have videos showing a child install one and it really is that easy. They even give you a multi-tool screwdriver to install it as well as some cover plates to cover any wall damage from an old (rectangle style) thermostat (we chose to paint/patch our walls and not use the cover plate).
You screw a small round mounting plate to your wall (which has a built in bubble level, cool!), pop your wires in (press fit), then the Nest clicks onto that plate. Simple, takes a few minutes at most.
It then powers on and you pick your WiFi network, enter your password, tell it where you live, and that's about it. Very simple and very intuitive. You then signup for an account at Nest.com and download the app for your iPhone (meh) or Android (yea!) smartphone, login, and you're good to go.
The app tells you the current temp, let's you turn it on/off, set temp, check humidity, etc. It has all sort of learning features as well so it can learn your schedule and auto set temps based on you being home or not. Yes, the Nest is watching you and knows when you are home or away (based on it detecting you as you walk by it throughout the day). These features aren't important where I live since HVAC isn't needed most of the time but these could come in handy for most homes and really save you money. You can override all of them (of course) and even set your own daily on/off/temp schedules just like you'd expect from any automatic thermostat.
Bottom line: Buy one today! It's a great thermostat and truly a well designed product and the wifi control works great. I don't have a single complaint about the Nest!