Well the downside is that that loses all value. That can occur for a number of reasons but the two most widely speculated on are a global response (something like the G8 deciding that they won't allow trading it) or a technical defect (which invalidates some part of either the transaction train or the currency itself).

But that said, its also very volatile as a currency and that makes for a lot of speculation and rapid market changes. Not for the faint of heart and certainly not a place for any money you might actually need a year from now.