As SEOPub pointed out, it is NOT illegal, I actually do it all the time and so do thousands of others. When I am running low on ideas for good articles and article content, here is what I do.
I will go out and find a series of decent articles, all touching on the same subject matter, of course, and I will read them several times... taking notes while doing so. I will then take the ideas and notes and formulate my own articles...
In theory, I'm writing my own NEW articles, however, in all respect to the original authors, I am merely rewriting what they have already said, and simply adding in some of my own additional thoughts (while even omitting some of the original).
Hundreds, if not thousands, of article writers and authors do this all the time.
It's NOT illegal, it's NOT unethical, it's NOT wrong. It's RESEARCH.
As Mike01 pointed out as well, COPYING an article, would not only be illegal, it would be unethical to boot, however, when done properly, you are only doing what thousands of others do every day...
How many times you have read different articles only to come out with the same core principles. It's just different people putting their own spin and perspectives on the same stuff...
The key is to add your own personality into anything you write, THAT is what will make your article stand out over others'.