At this point, Knightfall gets most of its worth as an historical artifact. By which I mean that it isn't really very good, but it is an important even in Batman's history. (Which, post-Flashpoint, may never have happened after all.) The whole Knightfall storyline (Bane breaks Batman, Batman comes back) was the starting point for The Dark Knight Rises, and this volume covers what the movie (thankfully) chose to ignore: Bruce Wayne's less-than-worth successor to the cowl, Jean Paul Valley.
The thing is, Jean Paul's descent is compelling, on paper. But the execution... He falls so far, so fast, that Bruce starts to look stupid for even considering him as a successor. I get that the storyline as a whole was a "take that" to fans who wanted an even darker and edgier Batman, but Dixon needed a much more subtle hand to make Jean Paul's progression (regression?) in the story believable. As is, he's about as subtle as a two-by-four.
Nothing else here is really worthwhile. The original villains are dull and uninspired (cowboy twins? wow) and the established ones aren't used in interesting ways. And frankly, the setups used just aren't enough to hold my attention. I mean, this is a monster of a book (650 pages, or 27 issues) and it makes reading it all a slog. The art isn't that great either. Really, if it weren't such a pivotal event, it wouldn't be worth reading at all.
Oh, and one final note: the summary here on Goodreads mentions Nightwing, but he seems to be playing Sir Not Appearing in This Book.