Do you, as a GM, reward players for having a detailed and well-thought-out background?

I do. In every game I ever GM, be it D&D, Mage, Shadowrun, or whatever, players get bonus xp/karma/whatever based on detailed backgrounds. They also get bonus xp for what we call "bluebooks", or little stories of what your character is doing outside of game, or their reaction to an event, or whatever. I award based on quality of the product I receive, and whether it advances the character at all, or helps me as a GM better understand your character. Writing the same story with slight variations results in diminished or no xp.

I award contacts, qualities and sometimes gear.

Yes, definitely. It normally amounts to handful of karma for a few good paragraphs, but I recently had an player submit an 1,800-word essay written as a dating profile. Good, solid answers to the 20 Questions (or the 50 Questions) are also worth karma.

On the other hand, depending on the mix of the group, it can be potentially 'unfair' to penalize the guys that aren't quite so good with the word diarrhea. I'd recommend a cap, so even if you write 20 pages you can only get the same amount as the guy that maybe answered the above mentioned questionnaires.

I've been lucky enough to also be on the player end of this. My largest contribution was a 25k word novella on the character's background. It netted me over 150xp in oWoD, so I could catch up with the other players.

I've done but had a problem with a few players just doing the bare minimum to qualify and it became a pain in the ass to go through things and make sure they made sense. It's bad enough when it is just a few paragraphs. When it is a whole story and you need to proof read and edit things so they actually made sense, it wasn't worth whatever benefit I might get as GM. And it still doesn't prevent the guy who has no loved ones and nothing he cares about. You just end up with more detailed reasons for that behaviour.

I let them have stuff/karma if they gave me a good background story (or at least a good set of 20Q without forcing them to hand me one)

Pity I get the TLDR treatment from my own GM

True, Mantis. For a person to write a large backstory, they really need to know the world to make sure it fits, and the limits of what their character could do, so they don't go crazy. Luckily, I was aware of both for that crazy huge story.

/me got Karma for having the character go to the loo once . .
Because nopony ever thought to do so.

Yes, but was the strain physical or stun?

As a Troll, for any lesser Character it would have been physical i guess.

ShadowFlush™

It's a dirty job...

Ok, I've capped the absolute max people can get from background for my game:
10 karma. (Easy to get, you need to fill out the question sheet I handed out [from that thread. thanks again guys] for 5, then give me how you came to run the shadows and why for another 5)
10,000 nuyen (Where you got your skills and resources, possibly details on another mission you performed before joining the group)
7 points of contacts (For mentors or other characters mentioned in your background)
Any positive/negative qualities I decide fit the background. (I plan on capping at about 7 points worth, though I may be more generous with the positive if I'm also giving out negatives)

I award Karma for background info. Usually 5-10 Karma (or so).. depends on how good the info is. The more hooks they provide, the more I give. I also reward gear and contacts as someone else said earlier (when it makes sense in the story). Usually I keep these low C/L contacts though. Same with gear. I don't give out anything really big.

Sometimes I make starting packages for new characters too. Like three sets of bonus skill ups, spells, nuyen, gear, and/or contacts. Also fun, but not game-breaking.

In my latest campaign, I've decided to give free contacts and knowledge skills with no real limit, as long as they're in the background and fit the concept of the campaign. In any case, it's up to me to decide if the contacts are available and how willing to help they are, so I don't worry about game-breaking.

But for most of the thing, I do it the other way around: I demand a few things from the players before we start to play and if they don't give them, they don't get to play. I require a well-thought character with a decent background and who will fit the theme of the game and the rest of the group. I also require details as to how they got their gear, powers and spells. If the character is a mage, I have the player read the wikipedia page of his tradition (at least), so that he knows what it's about.

I don't reward the quality of the background or the quantity of plot hook with karma or anything, but most of the time I'll care more about characters with an interesting background, and characters with more plot hooks are more likely to get the spotlight than character with no plot-hooks, so it's still to the advantage of the player to make a nice background.

Well, for my technomancer my GM was super nice and let me submerge at chargen. I wrote a four page story on the experience, including a Resonance based AI (wanted to set it up incase the TM got some paragon lovin' in the splat books). My GM started laughing maniacally and offered me a full ten karma if I promised not to mention him in the rest of the backgrounds and didn't tell the group about him... I took the points and he left muttering about Elder Resonance AI like I had just handed him Cthulu... I hope my group doesn't discover that I sold them out for two complex forms and a rank one skill.