Admittedly, I'm a 100%er, and with few exceptions, I'd agree with Yahtzee; it is NOT a lot of fun.
It is definitely a borderline OCD need to check everything off. When I log into my 360 and see games with unfulfilled achievements, it DRIVES ME NUTS.
On the flipside, I don't give a crudmuffin for gamerscore. So thankfully I'm not drawn to buying a bajillion games. I just have problems letting go of the ones I buy.
I would love to go with Yahtzee's plan to cure my affliction, but sadly, my friends' sisters aren't all that... d'oh!