My take is that it depends upon what you mean by "his work."
In terms of the accuracy of his polls, sure. I think he provides a helpful snapshot of what is going on in reality.
In terms of his theological and even cultural presuppositions, I'd say be very wary. When he does analysis of his data and then moves towards offering proposals of courses of action... I think he's way off base.
his job is to gather statistics. He often places his methodology and even his forms/tools/questionaire's in his books (I think, if I remember right). His, his statistics are as trustworthy as the people who answer them I guess.
I would say that George Barna's poll's are highly informative and provide a good snapshot of the state of the church. Sometimes his analysis seems to be tainted by his opinion and perspective.
His polling info may be of some interest but I'd never trust it to guide a church or ministry; In my humble opinion trying to survey a very heterogenous group like "Evangelicals" in the US is going to be very limited in accuracy and utility.