I'll throw my two cents in. I've used it twice for live events. Once for the bowl game, once for signing day. It was great for the bowl game. Some folks were using it as their primary source of information because they were overseas and could not access the telecast. Others just liked to get the perspective of people there.
But on signing day ... now that was when it was REALLY useful. You don't have 80,000k of your fans in the stadium and another million or so content to just watch on TV, not needing to crank up the laptop for additional info. What you had was a niche fan base DYING to get as much info as possible. So a live chat was great.
Having said that, as others said, it demands your full attention. I would not have the gamer writer doing this. It's hard enough to keep up when it has your full attention. Being the gamer guy would make it that much more difficult. What I did, is invite people to ask questions to anybody in our coverage team (talking bowl game here) and when they did, those guys were right next to me so I'd verbalize the question to them, then type their answer, in quotes, to the questioner.
It was quite an experience doing that. When I saw this thread, I was actually sort of debating in my head whether to pull it out for a couple of big games coming up: A big baseball series and maybe the spring game. Unfortunately, the school I cover is so bad in basketball right now that I won't bother with that.