(Update Mar07, 2009) I found a good example on performance comparison of regular ETF vs. leveraged ETF. In the past year, the finanicals ETF XLF lost about 75%, while the ultra financial proshares UYG (2 times bull) lost 95%. You can use google finance to draw the comparison.
MorningStar link here.
More about ETF at Wikipedia. The bottom line is I feel those ETFs are not meant for long term investing, because of the leverage and their trading strategy.