So... have you done it? And in what games?
The only game I've caught them all in is Red. Started out motivated by the rumors that you would get Mew after catching all 150, but kept going even after finding out that that wasn't true. At that point, I was just doing it for bragging rights and maybe a part of me still wanted to believe that that rumor about getting Mew really was true. I ended up hacking for that Mew to get a full 151/151.
I was much less motivated the next gen, since that meant catching all of those pokemon again, or at the very least trading them over and breeding them, which is also tedious. Also, in Red I benefited from having almost every other person on the planet playing Pokemon, so that helped me fill out the dex and in later generations, I didn't have that luxury. I tried again in Diamond, but came up short since there were just so many.
But now... I've decided that I gotta catch 'em all. With a limited free period for Pokemon Bank, I knew I had to transfer over all my legendaries, and any other rare Pokemon that are hard to come by in X/Y. With all those Pokemon transferred over and the new Exp Share mechanics making it easy to mass level up Pokemon, I realized that it might just be feasible this time around. And the other night, when I ran into two shinies in one night, I was reminded of the big motivation to fill out the dex: the Shiny Charm.
So, anyone else?