Ok, this is more about the Nest in general than the unit itself. Replacing an old, and poorly installed, Honeywell thermostat, I decided on the 2nd generation Nest unit over the current 3rd gen one. Why? Cost and features. Our furnace isn't the latest and greatest unit, and wouldn't really warrant any additional benefits from the 3rd gen Nest. Using a Nest display as a wall clock is also not appealing. The extra $50 didn't seem worth it for no real added benefit. (That $50 could go into a Nest Protect or Nest Cam)
For those still worried about compatibility and installation, you can take a photo of the current wiring of your thermostat, email it to Nest for evaluation before purchasing.
After ComEd's electricity rebate of $100, and Northshore Gas' $20 rebate... the $200 2nd Gen Nest totals only $80. So it was an easy choice.
Installation is extremely easy. As long as your current thermostat is wired properly, it takes about 15-20 min. with the only tool needed being the included screwdriver. Although a powered screwdriver will make things go faster.
So how is it living with the Nest? Quite good. One tip, is to create a new Gmail (or agree on a common Gmail) for your Nest account. This way, every household member that you'd like to have control can install and monitor the Nest from their phone. This also makes the Auto Away (the feature that tells the Nest that you're not home) work much more reliably since it's tied into your Google+ location as well.
For the first 3 weeks, the Nest was playing around with the furnace. By this, I mean that it would turn it on as needed but shut down after a few minutes. Then start up again. This made me initially think that the furnace or Nest was broken. But as the days went by, it learned how our furnace worked and how long it took to heat the house, and when to start heating up.
Granted it has been a mild winter this year, there have only been a couple of times when I've had to adjust the heat up and only for a few minutes.
The Auto Away feature is quite useful. You never have to worry about leaving your heat on, and even if you do, a quick check on the app allows you to switch off your heat in case it hadn't detected you weren't home. It gets it right about 85% of the time so far.
Savings? YMMV. If you keep your heat at 85ºF (30ºC) in the winter all the time, then you probably won't notice much savings. So far, we've been quite happy with a 68ºF (20ºC) setting for early am and early pm settings and 65ºF(18ºC) for most of the day. The weekly report shows our furnace on for about 2-3 hours a day total. Probably savings of a few minutes every day. It may not seem like much but the remote feature and learning function are worth it.
All in all, it's worth it. If $250 is too much for the current model, get the 2nd gen model. The smaller screen isn't that inconvenient and is $50 cheaper. Rebates bring the total cost of ownership under $100. It looks good, works well, and is the easiest connected thermostat to learn.