I can type faster with the iPhone's keyboard and have a level of accuracy equal to normal thumbboards. Prediction is just icing on the cake. It's not the end be all; I can think of plenty of ways it can be improved too.
Don't knock it unless you've used it for a few weeks to a month repeatedly. Once your muscles are trained to the positions, you can stop touch typing. I'm pretty sure people find the 5800 easier at first because you have to type in landscape and the entire screen is taken up by the keyboard, but even as you get better you will always be limited with the word speed you can put on it and the fact that you will have to switch to T9 if you don't want to use 2 hands. I don't consider T9 a bad thing as I can type at a ridiculous rate with that but qwerty will always be better for entering web addresses and using the gps.
However, I wouldn't use the iphone or current touch keyboards for doing anything more than emails, texting or browsing. Though honestly I don't think I could type anything long on anything other than a real keyboard either..