Personally, I liked Freakonomics. By pointing out "the hidden side of everything," it forces the reader to think about how things are connected in ways the average non-economist might not imagine. However, that's all the book is really good for. There's not a lot of serious economic analysis in there, only interesting ideas thrown about at almost at random. Thus, reading Freakonomics for the conclusions makes about as much sense as reading Playboy for the articles. For this reason, a book responding to Freakonomics shouldn't have been necessary. Given the buzz created by Freakonomics, though, it probably was, and Lott is as good an economist as any to do that.
For example, Levitt and Dubner wrote in Freakonomics that realtors keep their homes on the market a little longer than their customers do, and also make a bit more profit upon selling them. From this, Levitt and Dubner jumped to the conclusion that this meant realtors are systematically scamming their customers. Lott rightly countered with a much simpler and more straightforward explanation: every realtor follows his own sage advice, but not every realtor's customer does. Lott's conclusions regarding the interplay between crime and abortion are a bit more questionable. True, the fact that the U.S. and Canada experienced a similar drop in crime at the same time in the early 1990s, while Canada's version of Roe came a decade too late to fit neatly with Levitt and Dubner's theory, does appear at first blush to be problematic for their theory. However, at second blush it's not clear how how problematic that factor is, given that the very few Canadians live more than a couple hundred miles from the U.S. border, meaning that nearly all could have easily availed themselves of Roe in the interim. Lott is on shakier ground still when he argues that legalized abortion caused an increase in crime, while citing data equally consistent with the view that some other factor, e.g., the sexual revolution, caused both the increase in uncommitted sex (with or without contraception) and the push for legalized abortion. Given the relatively short history between Griswold and Roe, in which Americans enjoyed a "constitutional" right to contraception but not abortion, it's not clear we will ever know which factor caused the other. That said, Lott does appear to have made as strong of a case for the view that abortion causes crime as Levitt did for the view that it prevents it, thereby neutralizing the abortion-as-crime-control argument with which Levitt himself stops short of fully endorsing.
Lott has plenty of books filled with tables, linear regressions, and all that other geeky stuff few of us could understand if we tried. Freedomnomics is not such a book. Rather, it reads as exactly what it purports to be: a conservative reply to Freakonomics. Like Freakonomics, it is written to be accessible to the layman, not to provide the reader with a mountain of data supporting every conclusion. For this reason, I'd urge readers to read both books in much the same way: use both to help you recognize issues you may otherwise not have noticed, but take both books' conclusions on these issues with an appropriately sized grain of salt. And I'm saying this as one who agrees with many of Levitt and Dubner's conclusions, and almost all of Lott's.