I've invested heavily in some of the double short ETFs such as SKF, SRS, QID and so forth. They're all very volatile so you have to be willing to stomach the volatility. For now, I'm holding them and not trading them.
You should be aware they all have some error in the tracking of their underlying index. They tend to overshoot upwards when people are quickly piling into them, and overshoot downwards as people sell them quick. The "double" shorts also suffer from a quirk known as time decay, whereas over long periods changes in the ETF tend to get slightly nullified - it's nothing about the ETFs themselves, but more about the way the math is applied to the price series.
For example, the dow jones financials index is down -30% over the last 12 months while the double short SKF is only up 30% over the same period; time decay and tracking error has eaten away all the "doubling". Tracking error however can go to your advantage as well, at its peak SKF actually had a +143% return over the last 12 months whereas the index was only down down 45%. The extreme downtrend in SKF that ensued afterwards just killed SKF's advantage, for now. Double shorts are blunt weapons.
I also have puts on individuals banks, puts being a lot less "blunt". But the timed nature of puts means if markets stay "irrational" for long enough I could lose 100% of my investment even if I am proven right in the end.
I think you need a whole lot of conviction to bet against the banks right now - the problems are widely known and it's only a matter of how bad people things are going to get in the mind of people. I have the conviction, but it still doesn't make it easy.
Though frankly the entire market seems rather hard. Commodities are one the few remaining asset classes that is still up year-on-year, but the trend is definitely not their friend right now.