Hordes is a worthy expansion with a grand single-player campaign.
With a larger scale of assets, the campaign is significantly more lengthy too. This time round, there are lots of grand battles to let your massive spells feast on, though the experience points aren't much. The strange thing is, as you reach around level 25 onwards, the amount of experience you get per kill drastically drops, to the point where an "impossible" rated monster nets you only 200 xp out of the 20,000 xp climb per character level. That means... you don't actually get to level 40 in your first playthrough, which sucks.
Irregardless, Hordes of the Underdark is still a great game to enjoy in its glorious portrayal of epic-ness and tightly knitted challenges.
KingLim