You won't be able to gamble this time around however because they don't take credit cards or paypal from Americans... you have to send them a check through the mail.
As far as their accuracy on presidential elections... for the price of a couple of local TV ads their predictions can be easily manipulated. Plus you have to look at who the majority of “investors” actually are. Most of them are likely Europeans from socialist countries who favor Obama by a wide margin. So although they may be accurate on some things... I wouldn't lose a lot of sleep over their “predictions” for this election.
Clearly the amount of money involved would be pocket change to a Soros who wanted to manipulate. The only question is why is this the first time he (or whoever) would have done it?
Most of the predictions are only going to be as good as the available public info, such as polls and what not, much of which have been contradictory and/or non-representative. Though regardless, there’s more than enough ‘evidence’ out there—albeit of questionable epistemic worth—to cherry-pick whatever’s favorable to one’s biases.
I do imagine there’s at least a handful of investors on there with inside information from private polls, etc. who know the real score, but you won’t see any drastic movement in price from them until the last minute IMO.