Pro Integrates with numerous other systems
In addition to HomeKit, the ecobee3 can hook up with IFTTT (IF This Then That - an automation tool based on custom recopies), Alexa (Amazon's voice activated personal assistant found in it's Echo line of products), Wink (a stand-alone ecosystem) as well as SmartThings (Samsung's smart home ecosystem). This allows integration with a wider variety of smart home products outside of the HomeKit ecosystem.
Pro Remote sensors
There's no limit to the amount of sensors you can add through the home. They can detect motion and can tell when you're not home. It even learns not to confuse "no one at home" vs everyone is asleep.
These remote sensors also help to reduce hot spots and cold spots within your house by sensing temperature differences, which allows the system to correct for this.
Con Installation can be tricky for 4 wire systems
If your system lacks the common (C) wire and is only a 4 wire system, you will need to use the included Power Extender Kit (5-wire systems don't require this). Then, additional steps are required depending if your HVAC system has a controller board or not.