I disagree, I think he can and should be believed, painting him as a nut job is SOP.
Here's another example of where one term has a very wide range of possible meanings .. "nut."
I figure Burkett is sane, in the clinical sense, but obsessed against GWB. Similar to Mary Mapes in that regard, and Dan Rather, too. You can believe some of what they say, they are not literally delusional, and a good default position toward what they say is to assume it is a lie or a partial truth.
I'm sure Burkett has lied, as has Lockhart, Mapes, Rather, etc. The chore is to figure out which snippets of what they say are in fact true.
OK...so do we believe all of the crap that he says about Bush? No. The guy really is a nut.