Once you reach a point where you can do all the day to day stuff you need to in a timely manner then the diminishing returns kicks in. If a download takes 10 minutes at 10mbit then you'll notice going from 10mbit to 20mbit as it halves the time - you'll notice 5 minutes saved - but the gain becomes smaller the faster you go so by the time you get to the 5th iteration it's not significantly different to the 6th as we're now talking less than 10 seconds saved. Obviously in some applications 10 seconds is massive, but not usually on a home connection.