I've used Swype, SlideIT, and FlexT9 and have settled for the moment on FlexT9. I really prefer the suggestion bar to the pop-up style suggestions in Swype, and it's nice not having to squiggle for double letters and swoop down for the apostrophe in contractions. I do miss the numeric and cursor control keypads that Swype has, though.
I also use Hacker's Keyboard with ConnectBot for SSH, since it allows input of control characters and ESC without a trackball, which my current phone does not have. It works quite well, but the keys are too small for use on a regular basis.
Finally, I also use Inserty to facilitate entering frequently-used difficult-to-type phrases like my email address and passwords, as well as things like the current date and time.
If I add any more keyboards to my phone, my "input method" pop-up won't all fit on one screen any more!
