Interesting stuff, and quite comparable to Compete/Alexa et al. I wonder if they're using Google Analytics data for this? It doesn't seem so on first glance.
I couldn't find any of my site in the new trends search so it probably has a way to go before it's very useful.
Very interesting, now some personal data came from users are being used by Google for services like this.
The Google trends results have started ranking really high on Google now: http://www.seoco.co.uk/blog/2008/06/24/is-google-trends-for-websites-just-a-big-google-seo-ploy/
I cannot say that my site is very popular (around 450-500 uniques a day), but I found a lot of info about it there.
We wrote some parameters to use it in SeoQuake (FireFox seo plugin), so you can see all Gtrends data just in SERP or visiting site. (looks like alexa toolbar). You can get it here http://addons.seoquake.com/params/index.php?sln=en&browse=2&tag=google&res_lim=10