Robert Rapier has a post up: Is Matt Simmons Credible? discussing in particular some of his recent claims about the Gulf of Mexico oil spill, as well as some past claims he has made.
I am uncomfortable having to deal with this kind of thing: I have talked with Matt on several occasions, and emailed with him more often. He is a decent and well-intended man, has been helpful to me personally, and he has in the past had a track-record of good calls in the oil and gas industry, back before he got so famous. However, I'm afraid that Robert is now right: there is a history of sensationalistic overstatement in recent years, and since Matt is the premier peak oil spokesperson that can actually get attention from the mass media, that is a real problem. Robert shouldn't be the only voice to say so.
I think the most salient issue to me is the famous Tierney-Simmons bet, where Matt intentionally set the terms of the bet far out of the money. 2010 oil futures were $59/barrel back in 2005, so setting the terms at $200 was very disadvantageous to his chances of winning. It now looks extremely unlikely that oil will break the $200 level before the end of this year and so Matt will lose, and this does no good at all for the cause of getting people to do what is required to reduce their dependence on oil. It didn't have to be that way if Matt were a little less certain of his own rightness.