Tom Ricks' book really is as good as people say. You should read it. I'd offer to lend my copy out, except I'm borrowing it from the library.
One interesting point that Ricks mentions is that there is a generational transition going on within the US Army -- the generation of men who directly experienced Vietnam is retiring, and the generation of men who began their careers post-Vietnam are now in levels of high command. To Ricks, this is important because it means the current commanders -- and crucially, the commanders who botched the early Iraq War -- have an inflated sense of America's power. They have, after all, only seen America succeed at war -- Panama, Kuwait, Bosnia, etc. Ricks also sees this as leading to America's hubris-laden invasion of Iraq.
Color me unconvinced. First of all, the military officer most responsible for the Iraq invasion plan is Tommy Franks. He served in Vietnam as an artillery officer commissioned in 1967. His immediate predecessor at CENTCOM, and fiercest uniformed critic of his invasion, was Tony Zinni. He also served in Vietnam, commissioned in... 1965. Seems to me that's an awfully slim "generational" gap.
The simpler, though less attractive for a historical study, to simply say that a) Tommy Franks wasn't fit to devise a war plan of this complexity and scope, and b) even if he was, he wasn't the man pulling the strings. Don Rumsfeld would not allow a competent invasion to have occurred, because he didn't believe in the things that would have made the invasion competent (500k troops, counter-insurgency, de-emphasizing technology, etc.)
That's not to say that the people who dreamed up this nightmare weren't overly impressed by the events of Panama, Kuwait, etc. Obviously, people like David Frum, Richard Perle, Paul Wolfowitz, and the rest of the neocon gang saw victories like Gulf War '91 and incorrectly saw an "easy" win. But Ricks doesn't make a strong case that the Army has institutionally begun to forget the lessons of Vietnam.
Post-update: It occurs that maybe the Franks/Zinni comparison is inadequate for my argument. Zinni, after all, is a Marine while Franks is US Army. Zinni's predecessor, one J.H. Binford Peay III (seriously) is Army, and has also criticized the invasion plan, though he primarily blames Rumsfled (of course).