I personally feel that the Google Keyboard is underrated. It’s a perfectly good keyboard and is there by default on the Nexus and Google Play Edition devices. One of its downfalls was often that it suggested something incorrectly. Now that’s been fixed.
Previously the Google Keyboard would only learn suggestions from when you either backtracked on an autocorrection or manually picked a word that it offered. Now the keyboard is smarter because it can learn what you type into other Google services, such as Gmail, Google Search, Maps, etc.
If you don’t want this feature, you can go into the settings and disable it. And right after you install this update, Google will offer to automatically take you into the settings to disable this feature. But since the keyboard now syncs with the rest of your queries, would you want to?