My main computer since 2013, has been a rocking MacBook Air. It is the first generation of the system based on the Haswell chipset, and it has been awesome. It came with Mountain Lion (10.8), and has been upgraded thrice now, recently to El Capitan.
When I got it, one of the huge benefits was the battery life. The data sheet said 12 hours, and it easily beat that. I often went several days between charging the battery. I think my record was 14 actual operating hours, and I still had ~ 10% of battery left.
Of course, as time has gone on, I use it more and more, but until the upgrade to El Capitan, it was still rocking the great battery life.
I held off on the El Capitan upgrade as long as I could, but in a moment of weakness, I hit "ok" when prompted.
Unlike many of my friends, my upgrade went smooth and frankly, it was the easiest upgrade, and for Apple, that is saying something. Two entries of passwords (I have different iCloud and Apple Store accounts, a long story, don't ask).
The changes were fine, no issues, and since I don't use Apple mail, there wasn't any re-index.
However, there is a downside. The battery life seems to be about 1/2 what it used to be. Of course, the laptop is 2.5 years old, so the battery might be wearing out. But I doubt it is that drastic. El Capitan really appears to be a battery hog.