I overpaid £5k back in the first month as I had borrowed more than I needed.
I've two lodgers who's rent I save entirely to the sum of £835pm. I've unlimited overpayments so am just saving up, filling up ISA's etc and anywhere that gets me a better return (mortgage rate 2.75%).
Within the next 3 years it'll be gone, having just started it in October 2011, it's only a small mortgage. Including the initial overpayment it'll have saved me £18k in interest.
Chances are in 3 years time if rates are still as they are I'll use some of that to buy a new car, as it will be overdue, and a cheap rate of finance if you will.