Google Trends for web sites? No thanks
Here's why I stay away from this potential source of great competitive information, at least for now: I don't trust it at all.
I compared GTfWS data to our own data for the most prominent site for which I have solid web traffic info (but not Google Analytics). I analyzed seven months of our data, looking for a week-by-week stat, any stat, that would produce a trend line that resembles what GTfWS produced for the same period, same site. Unh-unh. Nope. No way. Zip. Nada. Neither the trend line or the quantities resembled each other. No stat I tried matched, certainly not the particular stat that Google Trends says it's showing - Daily Unique Visitors (per week).
I don't know from where they get their estimates. I know I've seen other people's similar analyses that corresponded very well - but they are sites that use Google Analytics. I know that my Google Toolbar connects to its home base every time I click on a page, even though I have "Send usage statistics to Google" turned off in the settings. Maybe these are related. Don't know, don't care. For competitive data, I'm staying away.