Works great with the 770 on your lap and using fingertips to stab at the letters - you can type quite fast that way.
But when holding the 770 in your hands, I've found a slight issue with it: fairly frequently it doesn't register a letter but still plays the 'tap' sound effect so you don't notice you've missed a letter until you're just about to press return.

While I'm here, I might as well run with it
I initially thought I'd really like to see a Tab key on the main page of the finger keyboard - so you can tab through input fields? Maybe there's no point though because you wouldn't always know for sure the tab order on the page was sensible...
They could get really clever about it and have the field name shown at the top (say in a small font?), so you enter your Name, tab through to the next field, which turns out to be Surname, then Middle name, if the tab order is a bit wonky etc.
That would depend on the site being at least vaguely accessible (in the DDA meaning)/standards compliant (i.e. with it's input forms properly labelled in the source with proper title attributes etc), but most sites make a good go at that these days, so it should work in most cases...
(Disclaimer: I test web applications for a living )