This is probably a bit subjective, but what do people prefer for pre-generated characters at conventions? Detailed characters with well-defined histories and personalities, or more sparse characters where you "fill in the blanks" to personalize that character?
My answer is, very strongly, "a bit of both."
It's very important to provide clear histories in order to ground the characters. In a con game your players won't know the characters, probably won't know the world, and may not even know or like the entire genre you're in. You have to provide enough information to let a total stranger understand how this character fits in to their world - because the player needs to have reasonable expectations for how the world and NPCs will react to them.
It's extremely important to provide enough personality information - possibly implied by the backstory, rather than directly stated - to give the characters goals. You don't have to say anything about the character directly, but you must provide an idea what the character would try to achieve, or how (which indirectly will say a lot about their personality). This is the number one reason I see for hesitant / unhappy playing at conventions - a player who doesn't understand what their character would want to achieve, or how to go about it, or has goals the player simply can't get behind. Give the player some idea what the character wants - other than just "complete the mission". Why does it matter if the mission is completed? Is there nothing else they care about?
It's also important to keep both these concise. You have a few hours for the whole game; the player has to read and understand the whole sheet in minutes. Anything that the player has to refer back to and reread simply won't get used at all. Don't give the character a two-page history of the conflicts of their family history; they won't remember the relevant bits when they happen. Instead, give them an NPC that they already know hates them, or single out one other pregen character as someone they owe a favour to. (One rule of thumb I use for writing: nobody remembers a character name or family name or historical fact until it affects a character they've met and care about.)
(Basis of comparison: this post is probably too much text.)
The reason for these rules of thumb is: in a con game you have no idea what kind of players you're going to get. And they may well be uncertain if they like the system, or shy, or not know the world, or be tacticians who don't like to act. You have to provide hesitant players with some clear guidelines to follow; in particular with a clear idea what goals to achieve and ways to go about them.
Players who want to "fill in the blanks" with character and personality are players who like to roleplay, so they won't be put off playing by having some provided background. Instead they'll elaborate upon it, using the material you've provided and taking it in directions you hadn't thought of. But people who don't like to improvise this stuff will be put off if they have nothing; some will feel like they have no idea how to play the character. Some people like clear structure.
(An aside: if people don't like playing certain kinds of background or personality, they'll generally say so when picking a pregen, or early. Be alert for signs of this in the first couple of minutes reading the sheets, and allow swaps or alternatives. I usually bring two more pregens than the actual number of players in the game.)
So to summarize:
There is not (or at least should not be) a single answer to this. It depends entirely on the type of con game you are running. Pregen characters are there specifically to feed the scenario and to communicate things that are important about that game.
If it's a roleplaying heavy scenario, like a Cthulhu Masters Tournament, especially where the relationships between the characters are expected to play a heavy role in the scenario, then the characters should be extremely detailed, up to a full page of background info. The detail should be game-relevant, however, or be important information the player should keep in mind when they RP the character, not random "facts" that don't bear on much.
If it's some random dungeon crawl, like any D&D Encounters scenario, a sentence suffices. Let the players make up more if they want. Here, the stats part of the character sheet is "what's important."
If it's a game with a nonstandard universe, say a Fading Suns scenario where you want to have a little character background to get across setting elements, you would probably have about half a page that is heavily seeded with "Joseph was sold to a Brother Battle momastery on the remote world of Isger when he was a spratling; he took Holy Orders and fought in his first conflict at the age of eleven" kinds of things that give players a quick-start on getting into that setting/genre.
I think it's best to have some background, but to overall keep it sparse. You want people to be able to read over the character in 30 seconds and have a good idea what it's about, so they can make a quick decision about whom they want to play. Probably the best kind of background to include is how the characters are related to each other - it keeps team work going. Another advantage to relationships is that in a con adventure with pregenerated characters, you can centre some aspects of the adventure around different characters. If a key character hasn't been chosen for whatever reason, you can still draw on that character as an NPC contact for one of the other PCs whose friend they are. Relationships with NPCs is similarly a good idea, as if a situation that stumps the party comes up, having an NPC who's friends with one of the party come in and drop some information in their laps doesn't seem entirely contrived.