I use them for the more conservative part of my capital.
I don't want the hassle of trading futures; limit up/down, notice dates, rolling contracts, etc. I can get access to commodities, currencies, and bonds with ETFs. I can even set a stop and go on vacation for a few days and not worry. Even though ETFs don't track these markets exactly, I don't find that to be a major argument against them. I am holding for 2-12 months if I can catch a good trend, so the tracking error is not very important to me.
More and more granular ETFs are appearing, and the liquidity on many of them is rapidly improving. There are at least 100 non-index ETFs now that I consider liquid enough to use.