It's been a while since I last played this expansion so some details are forgotten but it was Bioware at its best if you ask me. This expansion is what the original campaign for Neverwinter Nights should have been like with its characterization, customizable party members and choices that impact the plot. The writing is of better quality, the companions have more interesting tales to tell and there are multiple endings with a proper epilogue in the end not to mention the expansion features an incredible villain. As I've said so many times before, I think Hordes of the Underdark is on par with Baldur's Gate.
I never understood why both NWN OC and Shadow of Undrentide limited us to one party member either. I think they should have gone with two as well and the later could have especially done with this seeing as half of it was spent exploring dungeons and caves without much dialogue.
It's the shame Bioware couldn't go along with the original plot and features for the OC of NWN. I think it could have been as great as HoU if they had done. That sanity system would have been something to remember if it was done properly.