A great reason to consider ETFs is that they simplify index and sector investing in a way that is easy to understand. If you feel a turnaround is around the corner, go long. If, however, you think ominous clouds will be over the market for some time, you have the option of going short.
The combination of the instant diversification, low cost and the flexibility that ETFs offer, makes these instruments one of the most useful innovations and attractive pieces of financial engineering to date.