As we know, the best key words are in your mind in fact.You'd better spend more time on trying and comprehending to a nice product.Then you can think a nice adwords out.When you think about the adwords, you need to make a right direction of the words and use google tools to get related key words.After that,just test your adwords and have a try.Please be carfully when you have a test. Strictly speaking, there should be one adword in a group in order to observe the effect precisly.
You can search for keywords, then decide based on competition, number of clicks, cost, etc if you want to add them to your groups. It makes it very easy to see if a keyword is going to be too expensive to bid on and if there is too much competition, etc.
I find Keywordspy.com to be very helpful in finding keywords. Just type in the URLS of your top competitors and see the top organic and paid keywords they are using (free)
Yes it's a useful web site and I use it when I need to find keywords of competitors. It even shows ad variations which use competitors. Just type in a URL and you will see the great result.)
Even if a keyword is good for my competitor, it does not mean it will be great for me. It all depends on how my website is built and the keywords in it.
It might not be useful to steal their keyword but I find it more useful to be able to see what they are targeting so that I can target something different.