It's really nice to have another Weather API available. The only option I was considering prior to this was WeatherUndergrounds's API which is fairly simple to use. However, the pricing for Forecast's API is much better. Also, having a hyperlocal forecast for any lat/long is useful like DarkSky is.
Forecast's Pricing 
1,000 API requests per day is free. $1/10,000 API requests after that.
WeatherUnderground's Pricing 
You need to use their pricing calculator, and if you want historical weather, the price goes up a lot. $520/month for 5,000/API requests per day.
If you live in the Northwest, this resource, from the University of Washington, has revolutionized the exploitation of small weather windows for mountaineers in the Cascades. It's equally useful for choosing the right hour to ride your bike home, a day in advance.
Looking at the raw data sources, I realized, that for my location (Hamburg, Germany), the data on humidity from gfs really seems to be an outlier sometimes, affecting the forecast.io-mean. Maybe, just maybe the some other statistical measure would compensate for this and make the forecast a little bit more accurate.
Median, or something like trimmed mean.
And, as the service is using the location of the user, why not show the weather in °Celsius, if the region, where the user resides measures in °Celsius (and Fahrenheit, where Fahrenheit is used). I really had to look hard to find the button, to switch.
But non the less, great service, design, that I like, not too much info at first glance, but the most important one, imho.
Living in San Francisco for almost five years, I had to go back to my childhood in Massachusetts to test it. April Fool's day blizzard in Brockton, MA in 1997. Forecast.io shows heavy snow. I got like a week off from school. Works!
It lacks metric units (wind is in mi/s). I think that the contrast between ground and sea could be better; I had quite a hard time figuring out what region of the globe was actually displayed (I naturally first saw ground and sea reverted).
How does DarkSky accomplish this? Especially for a country like India.
I would like to start a weather forecasting service in my home country, India. Can someone here throw light on how I should go about it? Can I get any ready made softwares into which I can plug in parameters or do I need to understand many technical details and implement my own forecasting programs?
I guess I'll be the lone negative comment here... Personally, I find the mono iconography less useful than the ubiquitous blue/yellow/gray pictures. A cloud with two lines beneath means fog? Just not sure how that conveys it better.
Their summary is odd at times too. My next Monday shows: Icon: cloud with moon, Text: "Partly cloudy overnight.", Details: all clear
I really wish Android had better support for web apps. iOS supports creating a shortcut identical to an app on the home screen, indistinguishable from a normal app. Android, on the other hand, only has home screen widgets. Ideally you could throw a bookmark in the app drawer too.
Looks great. However, not being able to drag the globe around is annoying. From the UK, the global and regional maps are centred over Europe, but an overwhelming proportion of our weather comes from the Atlantic.
The top layer of our backend is node.js running behind nginx. We use redis for storage, along with a bunch of flat files for certain things. Under that is a mostly ruby / mysql layer that manages a bunch of C programs for doing various number crunching tasks.
Oops! That'll be fixed and pushed this afternoon. One of the huge advantages of this being a mobile web app rather than a native app is we don't have to wait more than a week for bug fixes to be approved by Apple!