I use Swype on my HD2 (Windows Mobile 6.5) and it is pretty good. However I do find I have to think ahead a bit more to get the spelling right so I can start my run around the keyboard. If you pause part way through it tends to take that as a double letter. Like Matt, two or three letter words often get done as taps.

It's also a bit more tricky to operate whilst moving (walking etc) although in those cases I probably shouldn't be trying to type something on my phone.