Siege of Dragonspear delivers on the most important aspect needed in a D&D game.
The combats in Siege of Dragonspear are challenging and fun.
Fighting the crusaders is fun. Fighting the varied monster groups scattered about the maps is fun. Fighting during the siege is fun. Fighting the final boss is ....challenging ..... but ultimately fun.
There improved AI that manifests itself around competent varied enemies of various types including spellcasters, thieves, and more make the game especially enjoyable combat-wise.
When I was much younger, in Throne of Bhaal, I would save multiplayer game files before big battles, Yaga Shura's army, Sendai's army, etc. When I wanted to re-live the battles, I'd load the multi-player game and delete old characters and replace them with new ones to have fun winning the battles in different ways.
Well, I did the same for Siege of Dragonspear. I had fun with with multiple different battles with different characters ( My F/M/T , Beserker / Mage, Cavalier, etc.) by loading multiplayer games and replacing the characters.
From the perspective of fun combats, Siege of Dragonspear is more replayable than Baldur's Gate. The epic battles there are just a few: Bandit Camp, Underground Temple outside area, Temple of Bhaal. Siege of Dragonspear has more.