It's quite possible to put together a totally scam project with pretty pictures and then waltz off with the dough after it's funded. I've never heard of it happening though, and if you're smart, you'll look at it as an investment. You go with the guys who've done it before that you trust, like the guys making Wasteland 2 (ala, they already made Wasteland 1). And unless you're filthy rich with a streak of altruism, back it with whatever amount you're comfortable losing.
In my case, $25 bucks is a reasonable bet for this game, since they've been going to trade shows (pretty legit), and folks I know who are in the beta tell me it's good.
Another thing to keep in mind is that when a project is funded, or crazily exceeds funding like Castle Story did, the Devs aren't getting all that money free. They still deliver the game to everyone who bought it. All it means is that a few thousand people bought the game before it was finished, thus helping fund the development. Much like M&B did in the old days. Exactly like M&B, in fact.
If the Devs are smart, they'll use some of the extra money to give the development of the game a little unanticipated boost, thus giving the backers the warm fuzzy feeling that they helped make the game better by their 'extra' funding. But at the end of the day, the backers still played $25, or $50, or whatever they did, and will still receive the rewards for that tier. But they are in no way obligated to go the extra mile if they exceed funding.