MyScript's Stack (free) is the "un-keyboard" replacement for iOS because it uses handwriting recognition rather than a standard keyboard-style interface. As you write, using either your finger or a stylus, Stack interprets your handwriting in real time, letter-by-letter, while also making word predictions. Like others in this category, those suggestions appear directly above the handwriting area, where you can tap to insert the desired word into your text.
I've had pretty good experience with this keyboard, but I've found that I prefer using it on my iPad, where it has a much larger writing surface than on the iPhone's smaller screen. However, size really doesn't matter in this case, because you can write each letter directly on top of each other on the smaller screen and it works just as well as if you write each letter consecutively.
While many of the most popular iOS keyboard replacements have been available for Android devices for quite some time, Minuum ($4) is a relative newcomer to both platforms, and its claim to fame is its minimalist design. In its regular mode, it functions as a normal full-size keyboard. (It is currently available only for the iPhone and iPod touch; no iPad-optimized version is available.) However, you can shrink it down to just a fraction of its normal size, which allows you to see more of your screen. Unfortunately, that also means you see less of the keyboard.
Because of my own minimalist sensibilities, I find Minuum very appealing, but getting used to the small key targets has been challenging in my limited use so far. I think have a ways to go before I can achieve minimalist keyboard nirvana.
At first glance, you might mistake the TouchPal (free) keyboard for iOS 8's built-in keyboard. But if you look closely, you'll see that many of the keycaps sport more than one character. For example, the "Q" key has a small "1" superimposed on it, while the letter "Z" and the "@" symbol share a keycap. Tapping normally on the key produces the primary letter, while sliding up on the key produces the alternate symbol. This lets you access a wide range of common letters, numbers, and punctuation characters without having to change keyboard layouts.
TouchPal uses those extra keyboard layout screens to provide a huge number of emoji characters and graphic symbols. If that's not enough, TouchPal also makes an attempt at being a swipe-friendly keyboard, but that feature is not quite on par with either Swype or SwiftKey.
It's quite likely that Smile Software's TextExpander 3 ($5) is going to make a large group of people very happy--namely, the existing users of the TextExpander text-expansion app on OS X and iOS.
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