I think that google trends should be used once you through the google adwords, because adwords is able to provide you all the possible combination of keywords.then you cna fine tune those keywords with the help of google trends
Yes, it is very helpful & extremely popular program that allows an individual to search for the interest of Internet users around the world regarding a particular topic based on a keyword or a keyword phrase.
This tool will allow a person to enter in the keywords and keyword phrases that they are interested in researching and the program will display information on how often that particular subject was searched for in the past.
I personally have used google trends as a form of assessing the online landscape. I think it is very helpful more from a macro than a micro level. I dont know how it can be used for PPC but I do think it is good to determine keyword contect for your site.