I am so happy to see this being released! Nin & Jormy hold a special place in my heart; I love this keyboard so much it's silly. It's awesome, you should buy it.
In my humble opinion:
I have tried iOS 8 Swype and SwiftKey, Fleksy back in iOS 7, jailbroken iSwipe, AltKeyboard, and OctopusKeyboard, plus numerous combinations of the smaller tweaks like SwipeSelection... (the rest I'm forgetting. I know there's more.)
On Android I tried all the big names, most mentioned above. Personally I found Google's keyboard to be the best. I loved Google's keyboard: it had the stable accuracy of the stock iOS keyboard, plus well-built swiping.
iOS Nin has trumped them all: nin's dual-swipe is so natural and "why isn't every swipe keyboard like this?" when you get used to it. Excellent premise: dual-swiping is absolutely brilliant. Excellent design: it's built with quality and love, you can see it. It is hands down the best keyboard I have used on both platforms bar none.
Edit to add to post #1's bug report, yes sometimes third-party keyboards will not appear when tapping into a text field. This is a known bug in iOS 8. Similarly there is another known third-party keyboard issue while creating Contacts: your cursor will jump throughout text fields. It's another bug with iOS 8 itself (even in 8.1.1).
Edit2
How many hours did you use this for?
Does it Feel natural?
Does it have a learning curve?
100WPM on my iphone sounds great, but if it takes me a year to achieve. I'll just pull out my Macbook.
I have been using it for the last year at least. Hundreds of hours? It feels very natural; more natural than most keyboards. It does have a slight learning curve: it can come off as confusing at first as it isn't as simple as single-swiping, which can potentially confuse some people. But it's incredible when you wrap your mind(fingers?) around it.