When I first played Red, I didn't care about my Pokedex, but from Gold onwards, I've tried to complete it. I've completed a Pokedex in Red, Gold, Sapphire, and Diamond (national), and I've completed the Kalos Dex in X. For whatever reason, I didn't bother as much in White--I guess it just seemed too easy with the GTS. (Trading helped me a bunch in Diamond, too.)
At some point, I plan to complete my National Dex in X, though I haven't been pushing on it lately. I really should get it done, as the Shiny Charm would be useful. This generation, though, I want to make a living dex of competitively bred Pokémon (or at least one member from each family), as that'll be more challenging than just completing my Pokedex, and it gives me a fun breeding challenge. I haven't had a ton of time to work on it yet, but I have between a box and a box and a half done so far.