I have read his articles for many years now. Most of them (mainly on Palestine) are interesting and (so far as I can tell) accurate. I would say that he is a respectable journalist, and probably a darn sight better than most of those in the mainstream media today. Doesn't mean I always agree with him, or his (usually rather left-wing) political views.
There was a time when journalists like Cook could make a living writing for a mainstream newspaper, but these days there is no place for his reporting on e.g. Israel in such places as the Guardian.