The report itself is well done, in my opinion. Given that an organization for "International Peace" is unlikely to come out with a report supporting war, I think that this report is surprisingly balanced. I agree with the vast majority of its claims, particularly their opinion that there was a massive intelligence failure regarding the quantity and quality of Iraqi WMD's. This failure was present in the British and Israeli intelligence services as well as our own.
There was likely deception by the Bush Administration above and beyond the intelligence failure. Interestingly enough, one interpretation is that the Bush Administration managed to deceive themselves. This is partly because the intelligence agencies slanted reports to be to Bush's liking (they were trying to please the boss) and partly because the intelligence agencies felt it critical to err on the side of caution. There was nothing for them to lose by being too shrill regarding Iraqi WMDs, but lots to lose by down playing the possibility if it turned out Saddam actually had them. As a result, I think the Administration is genuinely surprised, shocked even, that more WMDs haven't been found.
The Carnegie Endowment doesn't have access to classified intelligence documents. The report was based on declassified intelligence documents and other public information. There is, of course, some chance that classified information might refute some or all of the report. However, given how few WMDs have been found so far (or at least reported), it seems likely to me that the report's claims will likely ring true in the final analysis.
One last note. I feel the articles that referred to the report were less balanced. The report begins:
Iraq's WMD programs represented a long-term threat that could not be ignored.I don't think you would guess that from the articles. The articles happened to hand pick the quotes that were the most damaging to the Administration and ignored everything else (such as the focus on intelligence failures).