Here is a good article on ETFs: LINK
Personally, I stay away from ETFs as an investment vehicle. I have only used ETFs to gamble (money I can afford to lose) and when I do I it's always on the short side for a very quick (2 days max) trade. I play the short side b/c ETFs have intrinsic decay. I understand that with such quick trades that decay probably doesn't play a role but it's just psychological with me.