Are the 2 best places to do the research. The data I get back doesn't always match perfectly, but average them and you'll have a really good idea of what's going on with that site. Spend some time and drill down the links, it's really amazing the ideas you'll get when you do this. Some of the data you mine from this can be used to identify target markets. It's hard to overstate the value of demographics in marketing. The game is on when you know who buys what, and there are very real patterns that should not be overlooked.