Thanks for the comments guys:
Personally, I think the best choice is to just give the POV character a personality you choose, rather than trying to make it customizable. I mean, if you let the player choose, you'll have to riddle your code with if statements, trying to match the user's choices. My gut feeling is that will hurt the storytelling, while make it difficult to test all choices.
True it would be easier just to give him a personality from the get-go, but I think there could be certain advantages to allowing the user to determine, at a certain level, what kind of person they will be playing as. First I think it helps a player to have a sense of 'ownership' of the character, to better feel like he IS the character in some way - I'd allow name customization as well if it didn't make it difficult for Jessica to bop around calling him "Jiiiiiin-pooo" (I can't seem to think of a way to allow for all the different variants of that for some reason

). It also allows players to play a role they are comfortable/sympathize with... Some might find it amusing to play the joker type, while others might prefer to be all serious. Second, I think that the personality of the character should affect how the girls, at least at the beginning, would see him, and make it more or less difficult to attain their affections. A serious guy wouldn't appeal very much to Jessica, and a joker wouldn't float Cynthia's boat... you can still go for them no matter your type, but it does make for a different dynamic. Which brings me to point three, which would be added replayability.
But yeah, I'm ending up with a lot of if statements. ^^;
Another issue is that the very start of the game is one of the worst places to put a choice. The player knows little of the world, POV character, or heroines... so he's really just clicking randomly.
Well, he's not really supposed to make that decision based on that context, but more on what type of character he'd like to be, which preferably might be someone who is very much like the player, to add to the immersion. Sort of a very simplified version of those tarot card questions which started out the old Ultima games?
Do you want the personality to influence the game or just be flavor? I haven't played Mass Effect... does your "past" influence how the character acts?... or does he act based on the dialogue choices you pick?
Well, more flavor and some minor influence - actions speak louder than words or personality. The personality would come into play largely in terms of dialogue option availability (with corresponding flavor text), and occasionally the availability of certain actions (not all characters get to go around sneaking peeks at the ladies for one) but nothing that would directly branch off the story - though they will affect relationships of course. (Peeking at certain ladies could mean game over

)
[As to ME, you get to pick a background story that is referenced throughout the game, and results in certain quests, but your personality is determined by actions you take, and these open additional dialogue options rather than changing the outcomes of already available options]
The thing about Planescape is that it recognised intent as well as words.
*nods* I'll try to have some of this, but as expressions of personality more than as determinants. The joker personality gets to do sarcastic wisecracks, while the angry personality-type gets the option to punch the offender in the gut.