In portrait mode, you can switch between regular QWERTY, compact QWERTY (two letters per button), and T9. I prefer T9 for portrait typing since I can never seem to type accurately on those tiny little portrait QWERTY buttons. It also has voice-to-text and it also seems to be better at prediction than the stock keyboard. Also it doesn't hurt that (IMO of course) it looks nicer than the stock keyboard and has more configuration options.
The way I see it, it's not that different from stock but it has a few advantages, so why not use it?