You got it! These are not long term "buy and hold" investment vehicles due to the decay. You can make some money trading these, just make sure that you use tight stops to control the damage. I forgot to do this and got my ass handed to me, and then compounded the damage by chasing it down. It happened, will not happen again. I now use stops on every trade.
As you can see by the chart these things can decay. But I've stayed in FAZ for a couple months between November and January and came out ok. This particular time I'm in deep but this thing can turn around and make the money back, even with chasing it down.
The problem I have with setting stops is that I re-invest after it has gone down further, thinking it will eventually go up. For example, let's say I buy at 40 and set a stop at 30 and it hits so I lose 25%. So I figure I'll buy when it hits 20 and it does only to go down to 15 and I'm out another 25%! These things don't go straight down either.
It's tough to know when a stock will go up or down. FAZ was going up in the mid $30's until Geithner promised to buy out toxic assets. Then the price goes down 4 or 5 dollars over night after the markets have closed. Who knows if it will continue going down when the market opens in the morning?
One big reason I do invest in FAZ is because I believe the sector (Financials) still will get worse. When it continues to get worse FAZ will go up.