Horwitz is a mix between Bill Bryson and Ken Davis (author of "Don't Know Much About History"). He's amusing like Bryson, but develops his thesis like Davis. That being to show how little most of us know about who was "first" to America. You thought it was the Plymouth folk? They just got the best P.R.
I'll admit the section about De Soto and Coronado's travels were a bit dull. I mean, they travelled across the dusty Southwest or the swampy South ad infinitum. And the names started to confuse me, as well as keeping straight who was actually a good guy (de Leon, sorta) and who were bad guys (um, everyone else). But the Viking portion, the description of the sweat lodge Horwitz attended, the St. Augustine section, and eye-opening Jamestown that I guarantee you are not familiar with-- all these are well worth the read.
He does try a bit too hard to debunk popular opinion, although he makes up for this in the last few pages. It's a mite forced, and doesn't do enough to change anything he stated in the previous 12 chapters. In the final analysis, it's overlookable.