Most of the entries included in the book are solid articles, but most tend to merely skim the surface of the topic at hand, and I would love to go deeper and discuss the posts with the HN community.
They are brief enough to have something like one per week, and not distract too much from our day jobs (no more than HN itself, anyway). Also, I believe all the entries are still freely available at their original sites, so we don't have to worry about people not participating because they could not find the book.
As for downsides, I think a couple of the posts were submitted to HN before, but I think most, if not all, of them are worth revisiting. Also, it could be the case that many members have already read the book, but again, I think these posts are good enough to read again and discuss.