Overall, Loewen's opinion on Bailey, et. al.'s book is the same as his opinion of most American history textbooks. That is, he thinks that it is full of facts, some of them wrong, instead of giving us ideas and analysis. He thinks that it tends to make American history look good instead of looking at things critically. It is, in other words, like all other history books he looks at.
For a more detailed look at his opinion, you would need to look in the index and then at each mention of The American Pageant. You would find that he sees some good things and some bad things in this book as compared to other books. For example, on pages 45 and 46, he says that this book is the only history book he looked at that still claims that Columbus's sailors thought the world was flat. On pages 213 and 214, he says the book's discussion of American intervention in Haiti is more "naive" than that of other texts. On the other hand, pages 215 and 216 tell us that The American Pageant has the most accurate account of American intervention in Iran in the 1950s.
So, Loewen thinks that this book is just as bad as the other texts, but he also discusses more specific positives and negatives of the book at various points in his work.