I'm very impressed with this - it's a great idea.
Unfortunately a couple of sections of the Underground do not have the details to accompany them - the entire East London line and the Jubilee line east of Bermondsey. There are a couple of innaccuracies (where it recommends the wrong carriage) and it is slightly out-of-date in some of it's information, but it's still a useful starting point - you can fine tune your positioning with practice!
It's nice and small, when using it on the underground, and it will save you lots of time, especially during the rush hours when I'm sure you'd rather not be stuck behind two hundred other commuters all trying to use the same escalator at the same time.
Recommended for anyone who uses the underground on a regular basis.