Okay, I am going to try and give you a balanced opinion on this, using my own thoughts, conversations with priests, traditionalist viewpoints, etc.
Dr. Hahn catches a lot of flack for having been protestant. Personally I think that's retarded, he's been a Catholic for what, over twenty years now? My wife was Catholic longer than that and is just now doing RCIA. When are you "Catholic" enough? Where's the magical timeline?
Dr. Hahn was instrumental in my own conversion, and although I have moved past his fluffier works, I still appreciate his more indpeth works and particularly his revised dissertation on covenant. He is a formidable theologian , but he chooses to write accessible works for your average cafeteria Catholic to deepen their faith. Most of his more scholarly work people haven't even heard of, and that's fine. For those that are so inclined, his more accessible work often leads them out of the kiddy pool and into deeper waters, and in that sense he is a true blessing.
Now for some constructive criticism. He tends to have the "shiny new toy" syndrome. It doesn't bother him if only a few if any theologians have ever interpreted a passage as he does, because he seems (some would contend) to look for a novel idea, and then present it as fact. A lot of his ideas ironically come from protestant scholarship, and if you look at some of his cited sources, few of them are Catholic. Many argue that this isn't a good thing, that tradition is tradition for a reason, and if it isn't taught by any fathers or tradition, then there's probably a good reason for it.
As a theologian, Dr. Hahn has every right to pursue scholarship in areas that have not been defined by the Church. I would prefer a more cautious approach in light of tradition, but I can't blame him for wanting to trailblaze. Here's the problem though: precisely because most of his works are for your average layperson, they should be learning what the Church has always taught, not private speculations of Dr. Hahn. If a layperson feels so moved, then he can further look into matters in more scholarly works. However, the average person has neither the time nore inclination to do so. Dr. Hahn's works, for better or worse, may be this person's main exposure to Church teaching (the term "Hahn-verts" comes to mind). It should be Church teaching. Dr. Hahn seems to have the tendency, even in his more orthodox works, to promote his own theory, even as he admits it is not the traditional view.
I'll give an example. The new Ignatius Catholic Study Bible. I have it, I love it. In the book of Galations, he talks about "works of the law." The Church, along with thelogians, fathers, and the Council of Trent, has taken the hardline view of "works" referring to any moral work of Torah. Law does not justify apart from grace, and circumcision is the main example of a deeper spiritual issue. Dr. Hahn prefers the "New Perspective," which states that it only refers to ceremonial works like circumcision. Because Dr. Hahn prefers this position, he cites Jerome, Origen, Ambrosiaster, Aquinas, etc in support of his view, to make it sound like it is very old. He does admit that it is contested, but only gives one (Augustine) to the contrary view. This is hardly equal play. Here's the thing though, New Perspective on Paul was started in protestant, not Catholic scholarship. And I looked up every single example he cited, and they were all cited selectively. Ambrosiaster refers to Law in both ways, implying the traditional broader view, as did Jerome, etc. And even if Jerome did think so, his commentary on Romans admittedly took from Origen, who is not the best witness for orthodoxy. I love the writings of Origen, but you need to exercise caution. More importantly, even though Catholic theologians at the time of Trent espoused the "ceremonial" viewpoint, Trent did not use such argumentation, and this is significant for our situation today.
Other examples could be his viewpoint on the millenium in Revelation. He admits that it is not traditional, the Church understands the binding of Satan as being on the cross, not the Old Covenant under David. He can only point to one Catholic theologian named Corsini that has ever believed as such, and yet that's the only viewpoint you see about the apocalypse in his books, Michael Barber's etc. He goes on to say that this viewpoint is completely compatible with the trditional view, a la polyvalent prophecy; the four senses of Scripture, etc. However, this is almost a brush-off, since he never shows how the two views are compatible, and never even teaches the traditional view. Similar things can be said about the idea of "uncovering Noah's nakedness," the actual nature of the Original Sin of Adam (absolutely no traditional support), and some of his views on the Holy Spirit (which I don't personally have an issue with).