We bought an ecobee (ecobee 3 lite) last week to replace one (of two) old Honeywell thermostat.
The Homekit integration is very nice, and setting up a schedule was a breeze (I'm not one for the "smart" thermostat auto-scheduling, primarily because we have eight people and three dogs in the house all with wildly varying schedules so I have no confidence a thermostat would ever make sense of what we want when and also couldn't rely on motion sensors since the English Mastiff looks just like a person to any motion sensor). And, unlike the Honeywell, it was also a breeze setting up a "peak electricity" cool-down/warm-up cycle to avoid using the AC during the more-expensive peak energy times of 4-9 weekdays. So far, per PG&E's website, we're saving about $5/day compared to before the ecobee, but the temps also haven't gotten quite as hot this last week so that peak-energy avoidance strategy might end up not being as effective on a real scorcher where the house heats up too quickly.
All that said, ecobees are much cheaper than Nest, allow for "remote sensors" in any number of rooms you'd like to build a more complete efficacy picture in the thermostat, and allow you to use Siri to interrogate your thermostat ("Hey Siri, what is the temperature upstairs?" is the most common evening query in our house, replacing me walking up the stairs and shining a flashlight on the unlit Honeywell's screen to see what it says) as well as adjust the schedule when the last one leaves / first arrives home with absolute ease. The app is great when we don't want to talk (I'm still not a big proponent of talking to devices). We don't have enough other Homekit stuff to really benefit from "scenes" and such, but that is something I'm still experimenting with.
One other difference with ecobee is that you generally don't set specific temperatures on the device, but "comfort settings". You have three: "home", "away", and "sleeping". Each comfort setting has a minimum temperature (for when heating is on) and a max temperature (for when cooling is on). This indirection makes it easy to set things up so that it is a little cooler (or a little warmer) at night, without having to go to each day's schedule to set a specific temperature each time. That said, we have to reuse the "sleeping" comfort setting at the 3-4 hour to hyper-cool the house, and the "away" setting from 4-9 to allow it to heat up as needed. It took a little getting used to, but overall I like ecobee's approach.
The second (downstairs) thermostat won't really pay for itself until we get to heating season, so we won't be upgrading it until later in the fall. That one might end up being an ecobee 3 (non-lite) which includes one other-room sensor, and we might get a few other sensors to go with it (if someone is in the downstairs bedroom we might need to run the AC/heat longer to keep it comfortable, same with the office and potentially the front room).
Overall, without having seen anything other than Nest's marketing materials, I am quite happy with ecobee, and it integrates well with our iOS devices (and supports Alexa etc, for the Google/Amazon folks).
