I really think SLI is a waste of money. You don't get enough of a performance bump and you get all sorts of wierd compatibility problems. Not worth it unless you are doing rendering or some other really GPU crazy stuff
That's how I feel but there is a hardcore slice of the gaming world that feels differently. It doesn't seem to be a bad upgrade solution if your single card isn't cutting it anymore and a matching card has become dirt cheap because it's a few years old but I've never gone down that path.
The majority extreme gaming types have more dollars than sense. That said, I'm sitting on two Titan Xs, but I use them primarily for crunching, because they're much cheaper than cards marketed for that kind of work. For that reason I like the computer ricers.