I finally viewed all of the "new" commentaries, extras, etc., that have been added to the Blu-Ray version of the Trilogy boxed set. They're pretty minor--more paens to the Trilogy, by well-known actors (Jon Turturro, Joe Mantegna, etc.). The commentary is pleasant and erudite, especially in making the point that the Trilogy is about a family, not about the Mafia per se, which I've always believed. In "the movie that almost wasn't," FFC and others review jumping through hoops with Charles Bluhdorn, then chairman of Gulf+Western, which owned Paramount. Familiar stuff. "Four Short Films on the Godfather" is what it says--trivial, although one has some good GF dialog done by actors imitating it, another with FFC explaining that "until the point that I actually put a black armband on Michael V. Gazzo [at Anthony's party], I was still negotiating with Richie [Castellano]." FFC says the dealbreaker wasn't money--it was Ardell Sheridan's insistance on writing Castellano's dialog.
The extras disc also contains all the extras from the DVD version, including the much better "Making" film(s).

All in all: the video and audio of the three movies are somewhat better than the DVD, and the Blu-Ray-only extras are interesting to a diehard Trilogy fan. But they don't necessarily justify the price if you already have the DVD version.