It took me a while to get into the habit when I first moved to Sweden, but after a couple of years I did start using all my weeks. Having a really long (4-5 week) summer vacation really really helps with recharge and coming back more fresh and productive. Some people do end up with days rolled over into next year, but there's a limit to that accumulation after a few years, so if that's happening eventually managers and leads strongly encourage you to use more of those days.