So far, somewhat on par. Maybe I'm getting an extra 1 hour or so, but I rarely run my computer down to that low anyway. Both 2016 and 2017 models are lasting me >7 hours on the occasions that I do need the longevity. But judging from my usual usage (coding, testing, VMs), 2017 seems to drain maybe 1% less battery per hour? Nothing significant.
Most of what has been improved to me are:
1. Less "your Mac is overheating" warning. Got that hourly with 2015, then 2016 reduced it to about daily, and 2017 I have only gotten the warning once when the inside of my car was super hot and I forgot the MacBook was in my bag running a test.
2. Smoother performance with a 4K display, especially for scaling higher than 1080P. Still have to disable transparency effect.
Otherwise, equal everywhere else.
Oh, and worth noting: my 2017 model has 16GB of RAM versus the old 2016 model had 8GB of RAM. More RAM draws more power so perhaps that's why my 2017 model is on par with my old 2016. Maybe 2017 with 8GB RAM will have better battery life. But I certainly wouldn't want to find out considering 16GB is making my VMs very happy.