This is cool, but it might take too much effort for me. What could maybe sway me is a companion iPhone app where I could log as I go. Ideally it would all be voice activated and I'd just say stuff like 'lunch £6'... But even if you had a simple landing page where I could make single-tap entries to my ledger. They wouldn't need to be exact either, a button for 'pint' that guessed £3.50, for 'chocolate bar' that guessed 70p, a few ones for 'meal' (cantine/pub/restaurant), customisable ones for frequent train fares... You get the idea.

For me personally the main use case would be tracking what I spend money on, rather than knowing how much I have left.

On a side note, I wish my bank would get their act together and let me easily search my balance history on their online banking page. Instead I have to export it and the history gets wiped after 2 years or something.