Just updated to iOS8 on my iPhone 5S and installed my first third party custom keyboard apps, Swiftkey.
The stylish interface, by default two themes, black and white, built in smart prediction technology, and support type by sliding the keys.
Once installed the custom keyboard apps, will have a host apps put in the desktop. To enable the keyboard, go to Settings, Keyboard, add the third party custom keyboard apps. Once setup, go to any text box, click change keyboard button, until Swiftkey apps pop up. Start typing! Can anytime change back to the other keyboard.
To protect your privacy, when you enter password field, Apple will trigger back the default system keyboard. But for your own sake, when you enter another field or something else, that requires your credit card information, please, change back to default system keyboard yourselves if Apple cannot detect that.
In order to use type by slide, I must go to SwiftKey Keyboard Settings and checked Allow Full Access. But to check Allow Full Access, meant I allow the apps to be able not limited to, for instance, transfer the keystroke to their server. This is how smart prediction works. Transfer whatever we key in to their cloud, then their cloud learn our typing habits, recommend the words that we can put into our message typing. This is really not good if you really care about your privacy.
For myself, I will use Swiftkey carefully now. Once I learnt how to create my own custom keyboard with smart prediction cloud, then I will use it, for my own safety.