There's nothing quite like it. Checking your save profile to see a great big 100% gleaming out at you. As far as boasting goes, it's the gaming equivalent of pulling the Princess of Spain. As an achievement, it's up there with pulling off a Big Art Attack without the help of a grown-up. Getting 100% is the ultimate.
Yet hitting three digits is also a sad moment for all gamers. It means a game you presumably loved enough to pour hour upon hour into is finally over. Chances are you'll be struggling to motivate yourself to play this game much more alone.
Even worse are games that don't reward you for your feats. If you've just nabbed 100 craftily hidden boxes, bought every item in the shop AND levelled up as far as possible and all the game does is say 'Congratulations' or worse yet, nothing at all, then you know something's amiss.
100% is a very hard stat to come by in any of life's many percentagised disciplines. Missing nothing, every opportunity taken- Is the number alone worth the pain? Our editor TR would do anything to clock up a counter, while resident 'Extra' Gorrila is more motivated by actual stuff. New Game + will do, although extra modes and characters is what we want. A desire to see it all but not nesecerily do it all is certainly present.
Also lurking about is a piece of information that gamers love to cast aside. No other media form offers this kind of reward. Listen to a song, remix the song, play it at parties- You won't see your percentage increase, if anything the only rise or fall will be that of your bank balance as legal eagles take to the courts. You can watch The Human Centipede 90 times and yet you won't get so much as a 'Well Done' (even if a far stronger liver does seem a fair exchange). And books certainly don't give you Gamer Score Achievements for reaching page 76.
As a number, 100% cannot be topped- In all regards. Say what you will about giving 110%, it'll take it to get ten percent less. Beat a game and either move on or start again. Perhaps there hasn't been moaning about this subject, but sometimes a word of solemn is needed after beating a game as majestic as, say, Super Mario Galaxy 2. You'll never see another gold star again or experience the freshness that game had, but you still got to do it- And in such a fashion. We've arrived, but it was one hell of a ride, detours and all- Never mind your save progress, check your funometer. Still only at 99%? Just wait until you see that third digit for the first time...