The keyboard features a standard QWERTY layout that has been split in half and repositioned so that the two halves are partially stacked. This allows the individual keys to be larger and is said to let the hands/thumbs to rest in a more natural and comfortable position, which in turn results in faster and more accurate typing.
The keyboard lacks word prediction and auto-correction at the moment, but both are planned for a future update. Swipe gestures and some other options are expected to be introduced as well.
Long pressing the keyboard icon on the bottom left brings up a menu of options to change the layout and access other settings. The layout you see here is the QWERTY Compact, but there's also QWERTY Large and QWERTY Standard to choose from (screenshots below)
The odd layout is actually much easier to get used to than you might expect, but the tendency to tap certain keys with the "wrong" thumb can sometimes result in a little thumb ache from the extra reaching.
The number/symbols keyboard (bottom right above) is the same for all three layouts, though you probably won't need to access it very much with the split keyboards.
Preferences include keypress sounds, haptic feedback, adjustable vibration strength, checkerboard vibration (two different vibration intensities based on checkerboard pattern), colored keyboards (green and red outline around the keyboard halves), and auto-capitalize after a period.
I don't think I could use Thumb Keyboard as my primary input method (Swype and the new Gingerbread keyboard are just too good), but feel free to give it a try on your EVO.
Free version (not in Market, version 1.5.1, no additional updates):
Paid version (€1, version 1.8.2, all additional updates):
Screenshots from version 1.5.1 shown above. See my post on StreakSmart for info about the latest version, which supports tablets.
Need help enabling a different keyboard on your HTC EVO? Find out how to do it here.