I gave SwiftKey a try, and while I do like it and it works pretty well, I think Google's built in keyboard works just fine, making the extra download of SwiftKey unnecessary.
I used SwiftKey up until the AOSP keyboard implemented swiping a couple years ago, at which point I quickly ditched it. I find the AOSP keyboard significantly snappier and just as accurate, perhaps moreso, when typing swype-style.
I guess I'm one of those people who likes to use the default apps whenever possible, unless there's a really compelling alternative. In most cases, third-party replacements aren't quite as smooth, consistent and well-integrated. Google does a good job with most (though not all) of their apps.
Granted, now that Chrome has replaced the old AOSP browser, I find myself more tempted to use Dolphin/Firefox from time to time... but that's another story.