I've got the Badass dongle for offroad use and thoroughly recommend it. Zero hassle, works great, and is a ten second change to go back to road legal. I agree with the previous posts that the level of assist drops sharply over about 20mph.
Im 38, live in London, and bought a 2013 Haibike/Bosch 29er last year which I love! Finding the whole thing increasingly addictive and can't stop looking at the shiny new offerings with temptation :)