How well regarded is Howard Zinn's "a people's history of the United States" among historians?

Is it still being used or has it been discredited or found unfactual?

Essentially both.

Foner's take:

There are many grounds to criticize many of the things he wrote. I reviewed "A People's History of the United States" for the New York Times Book Review when it came out in 1980. I gave positive and negative points of view about it, but the point is, this was a passionate interpretation of American history. The way he inspired people, to me, is his legacy, rather than his interpretation of the Jacksonian era or the Gilded Age or the New Deal. Those can be debated and will be debated. But he deserves more than just people saying this is a biased historian. He really was an important figure in the public vision of history.

or Wilnetz

What he did was take all of the guys in white hats and put them in black hats, and vice versa. His view was that objectivity was neutrality, which I think is a formula for bad history. Objectivity is not neutrality; it is the deployment of evidence and building an argument based on historical logic. That's how we engage in rational discourse...He saw history primarily as a means to motivate people to political action that he found admirable. That's what he said he did. It's fine as a form of agitation -- agitprop -- but it's not particularly good history. To a point, he helped correct mainstream popular conceptions of American history that were highly biased. But he ceased writing serious history...every left-winger was always great. That can't be true. A lot of people on the left spent their lives apologizing for one of the worst mass-murdering regimes of the 20th century...You wouldn't know that from Howard Zinn.

http://articles.latimes.com/2010/feb/01/opinion/la-oe-miller1-2010feb01

http://www.newrepublic.com/article/112574/howard-zinns-influential-mutilations-american-history

there are some serious flaws in his work bu he inspired a lot of people and had an impact as a popular historian. It's not really a good source to build a strong argument on but it's a book i've seen recommended and loved by many.