It was the best weather API I've ever used, for a few reason:
1. Pricing. It was affordable and straight-forward pricing. There wasn't a huge "tier bump" - I think it was like a flat fee per 1000 requests after the free alotment.
2.You always could expect the same response, regardless of location or date. You could get an hourly forecast 3 years into the future, or 20 years in the past. Most other weather API will give you hourly only from "now" into the next few days. Or you can get daily for the next ten days or something like that. So you had to build logic around into your app. With DarkSky I got the same response no matter the day, time, or location. So it was much simpler to deal with.
3. Accuracy. I was always amazed at how accurate it was.