I will say it up front - I don't share the same level of enthusiasm for this episode that my fellow reviewers do. Yes, it's very good and introduces a scary new opponent, but the episode takes a while to get going, includes an uninteresting B-plot involving a new ensign, and overall the payoff wouldn't come until the end of the 3rd season. The shock experienced when first watching this episode doesn't persist (at lest for me) after repeat viewing. While it doesn't quite reach the level of "series classic", it does show the big leap forward the show took in the second season and where it would be headed in the 3rd.
The Borg's effectiveness, at least through the end of "The Best of Both Worlds", was their image as a sinister, unstoppable juggernaut. That ultimately limited their usefulness as an opponent/villains, since their mystique faded once they were defeated for the first time. However, after this episode the writers seemed to become more comfortable presenting other formidable villains such as the Romulans and Cardassians.
This was also among the best Q episodes. From this point on, Q would be presented largely as a sort of cosmic jester and a foil for Picard; but in "Q Who", he's a malevolent being and a clear danger to the Enterprise.
The best scene in the episode is actually the concluding chess scene between Picard and Guinan - as they digest the horrific events of the past day and realize the menace that lies ahead.