Sounds like a no-brainer to me. Any amount paid extra saves quite a bit in interest payments. I had always heard that paying one extra payment per year of a 30 year mortgage would shave seven years off of those 30 years. I never did this though. I don't know if this happens in Finland, but in the US, those interest payments (not principle payments) are tax-deductible, so you have to take the tax "savings" (taxation is theft) off your savings in interest.
I will have to look more into the tax implications here, as I don't know them in detail. Time to go to the tax website!! :D