That reminds me -- I've been meaning to add The Impossible State to our book list for North Korea. (Which I guess sort of answers your question in a nutshell.)

Short answer: Cha is regarded very highly. There's no uniformity of opinion in academia, so you're certainly going to find people who disagree with him, but most of this is over analyses of North Korea's future or interpretations of the Kim regime's motives. Even reasonable, educated people have a lot of room for honest disagreement on these topics, but I haven't seen any serious quibbles over the accuracy of his more history-centric work. The one caveat I would attach to The Impossible State is that it needs to be read as an American diplomatic perspective on North Korea and not an impartial view of the country, but in fairness, it was never intended to be an impartial view in the first place.

Long answer: For the benefit of anyone reading this thread who isn't familiar with Professor Victor Cha or The Impossible State, Cha isn't an historian per se -- his work spans political science, economics, history, and diplomacy. He's currently at Georgetown directing the Asian Studies program at their school of foreign service, and he's also the Korea chair at the Center for Strategic and International Studies (CSIS, a major nonpartisan American think tank). Most importantly for The Impossible State, he was the director (or deputy director -- I honestly can't remember which) for Asian affairs on George W. Bush's National Security Council and pretty much their go-to person for North Korea stuff.

From an historical perspective, The Impossible State is valuable because it's a portrayal of the Six Party talks through the eyes of a participant. Cha was one of the U.S. delegates, and the book was published less than five years after the final conference, when his memory of the events and personal notes were still relevant. It's rare to get an insider's perspective on major diplomatic talks, and especially at the level of detail that Cha provided. He was astonishingly frank about how he saw the participating nations (e.g., the Russian delegates' boredom interspersed with their cut-the-bullshit attitude with the North Koreans behind the scenes, what the South Koreans saw as Chinese overreach -- "It isn't their fucking peninsula, it's ours") and how their own particular national contexts led them to approach a problem. Some of the details are just slice-of-life bits that give you a glimpse of the human element in diplomacy, but a lot of them are the raw elements of massive geopolitical maneuvering.

The Impossible State was generally reviewed very well. However, the criticisms I've seen leveled at it tend to fall into three themes:

• Editing: The Impossible State definitely has some unnecessary repetition. Critics are correct that it needed more aggressive editing. This may be the result of the relatively fast timeline that the book had to have been on before publication.
• American-centric perspective: Some of this criticism struck me as a little odd. No, I wouldn't recommend The Impossible State as an impartial history of North Korea, but it wasn't intended to to be one. The book was pretty explicitly written as an American diplomat's view of recent North Korean history and the participants in the Six-Party talks. Cha gives a thorough account of each country's perspective, their historical connection to the North Korea issue, and what their likely motivations are. Personally, I think his chapter on Russia is one of the best parts of the book, and it's a fascinating (and sometimes eerie) counterpart to Andrei Lankov's work (a Russian commenter on NK). I don't think there's a strong argument to be made that the book exists solely to advance the American narrative.
• Academic politics: Being blunt, there are an awful lot of people in academia who will be unreasonably nasty to anyone who's worked for a Republican administration, and Cha is no exception. I remember his coming under fire in a few reviews for "peddling" the Bush administration's views both at the talks and in the book, which is a criticism I have rarely seen leveled at academics who worked under a Democratic administration. It's also a bit asinine, because people familiar with U.S. policy on North Korea will already know that the relationship between the two nations really started deteriorating in 1999. The worst thing that can be said of Bush is that he was following the precedent that an exasperated Clinton had set before leaving office.

Anyway! Foreign Affairs singled out The Impossible State as one of top picks of 2012, and I think that it was deserved. But again -- as with any other sources, it has its biases, and you shouldn't accept it uncritically any more than you should accept any other source uncritically. But Cha's legit, and it's a pretty unique perspective on the NK issue.