The way I see it, there are two types of expansions that can be released for a computer game. One is an expansion based upon adding more storyline and area to the initial game (such as Tales of the Sword Coast). The other is an expansion that adds enhancements to the original gameplay (such as the original Diablo I expansion).
Heart of Winter delivers both. And having played through the entire expansion myself, I am more than pleased with what I obtained for my $30. Addons that add enhancements to the original gameplay are uncommon, and providing both enhancements and further storyline/area is rare. HoW is reminiscent of expansions like the Ultima 7/Serpent Isle addons (such as the Silver Seed). You get more than just additional game time, you get improved gameplay for the original game. And if the additional enhancements are enough to make a person want to play the game from the beginning again (which HoW does, in my opinion), then you just added a lot more than just 20 hours of additional gameplay.
Having said that, I think that Heart of Winter is well worth your money. It brings the original Icewind Dale to a new level of playability... and for that, I am very happy to have bought it.
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Buck Satan
GameBanshee
Make Your Gaming Scream!