If you're not already familiar, TextExpander is a text expansion utility that lets you assign abbreviations to blocks of text and then expand those blocks by typing just their abbreviation. (See Macworld's previous reviews of TextExpander for Mac and TextExpander for iOS for a complete description of the software.) Released first for the Mac, then later for iOS, the Mac app works in almost every program, but its functionality was greatly limited in iOS 7 and earlier due to Apple's strict policies. Smile was able to circumvent these restrictions, in part, by providing a TextExpander "compatibility library" which other developers could add to their apps, allowing snippets to be expanded directly within those apps, just as they could on the Mac.
But now, with iOS 8 now providing greater inter-app compatibility, TextExpander 3 now functions as a third-party keyboard, which allows it to work across all apps on your device. That means I can now expand my email signature line in Mail on my iPhone exactly as I can in Mail on my Mac. That makes me very happy, indeed.
While you'll get the most value from TextExpander 3 if you also use TextExpander on the Mac, you can certainly use TextExpander within iOS only if you want, creating and managing your snippets solely on your iPhone, iPod touch or iPad, and even syncing snippets among those devices.
Sticking with Apple vs. going the third-party route
Apple is playing some catch-up here with the addition of this long-awaited feature. However, it seems like Apple rarely delays implementing a solid idea without good reason, and it's quite likely that security and privacy concerns were at the center of this decision to hold off until the company was certain it could be implemented properly.
That said, with the notable exceptions of Swype and Apple's built-in keyboard, each of the other keyboards mentioned here will not function properly until you enable a setting called "Allow Full Access." The "About Third-Party Keyboards and Privacy" link that appears at the bottom of the Keyboards setting screen describes that option and explains how much access to your personal data a keyboard app's developer is granted. So, before enabling any of these keyboards, you should read this info, as well as review the developer's own privacy practices publications, to decide if you want developers to have access to your data that's flowing through their keyboards. (Many of these keyboard developers openly disclose why they require Full Access, like SwiftKey.)
Similarly, if you have an iOS device connected to company email or other company-owned resources, you should consider contacting your company's IT department to see if they have any policies governing the use of these third-party keyboards.
Sign up for CIO Asia eNewsletters.