I love the Ecobee because of the remote temperature/occupancy sensors. I live in a house that has a separate HVAC units for upstairs and downstairs, so I have two Ecobees. Downstairs one has three remote sensors attached to it, and the upstairs one has one remote sensor.
My roommate works from home, so one of the downstairs bedrooms is his office. All of the equipment in his office makes the temperature in there about 5'F warmer than the rest of downstairs. The Ecobee does this "follow me" thing where it reacts off the temperatures of the sensors that sense occupancy. We have sensors in the living room, the office, and his bedroom. So if he's in his hot office during the day, the Ecobee uses that room's temperature for running the HVAC. If the Ecobee is set to 72'F, it'll run the HVAC until his office reaches 72'F, even if that means the temp in the rest of the downstairs goes above or below 72'F. When he's done with work and leaves his office, the Ecobee senses nobody is in that room and starts not caring as much about the temperature in there and follows to favor whatever other sensors sense occupancy. It's not magic -- he could be constantly manually adjusting a thermostat throughout the day based on the rooms he's in, but that's a PITA and not going to happen, which means the HVAC would likely be running needlessly a lot.
Ecobee solved a similar problem upstairs. Thermostat is mounted on a wall in a hallway that will never cool to 72'F on hot (95'F+) days. So if I set the temp to 72'F for upstairs, since the hallway will never reach that on hot days, the A/C literally runs all day, the hallway stays warm, and the other rooms upstairs freeeeeeeeeeeze.. Since I'm the only one that lives upstairs, I have a remote sensor in my bedroom, and it I set the upstairs Ecobee to always use that temperature, and to disregard whatever the hallway temp is. Now the upstairs HVAC only runs a little bit to keep my room comfortable (when it senses that I'm in it).
Regarding HomeKit, the only thing I noticed from that is that I can do Ecobee stuff with Siri. The Ecboee iOS and web apps work the same, regardless of whether you enable HomeKit or not. So even if this system didn't have HomeKit, it'd still be a huge win in my house.