and the Mentor Federalist Papers keep getting better. Yes, that's right. They actually managed to improve on it. The great new additions include the Declaration, the Articles, and an excellent new introduction by Charles Kessler. I think the killer feature for new readers will be the notes in the back, which, if you (like me) are shaky in your Greek history (and the finer points of European), do a great job of explaining allusions and references by the Papers. Be sure to use this feature -- there's no indication in the text that a note exists, but you should just look if you're unsure of a historical setting (or something similar), and there probably will be one.
On the minus side, I do miss Rossiter's introduction. It wasn't as good for laying out the plan of the work, but it should have been included (along with Kessler's) for its excellent overview of the contemporary situation and the philosophy behind the papers. Also, I feel that Rossiter's contents were slightly better than Kessler's. And, the page numbers are changed, invalidating older references to them. But all in all it's an improvement, and certainly the Mentor edition is the only one to have. Period. It's the one used by at least some of the Supreme Court Justices, and it retains that single dominating feature, Rossiter's cross-referenced Constitution (and index of ideas).
As for the Papers themselves, of course, they need no review. They are the first and ultimate Constitutional commentary, and fascinating reading besides. As literature they stand out for the exceptional style (all the more remarkable considering the haste in which they were written) and clear thinking, and more than any other book they define how the U.S. _should_ work.
All in all, this is one of the best book bargains on the market, that rare coincidence where best edition meets mass-market paperback. What are you waiting for?