Tulivu: Get ready for a huge wave of Planescape Torment fandom.
While I'd completely concede that PS:T is one of the greatest RPGs ever made (if not one of the greatest games ever made), I think that IWD never got the love it deserved from the D&D PC RPG crowd. Me, I loved it from start to finish, and even though I shelled out way more than I should have for MacPlay/OMNI's OS X port back in the day I gladly shelled out for it again on GOG.
I loved IWD so much because it gave me everything I loved about one of my other favorite games, BG2, but without the main element that really turned me off during my first playthrough. The combat was the perfect mixture of tactical and strategic gameplay with the fast and furious hack n'slash that I loved while as a pen-and-paper DnD player when I was younger...and it cut away all of the relationship BS that I didn't like in BG2. All of the characters were empty vessels into which I could pour in my own imaginary backstory, my own imaginary histories, and my own imaginary witty banter laced with innuendo-filled one-liners.
While many people complained about the simplistic plot, I was really engaged and sucked into it. The history of the Shattered Hand and the tragedy which befell it really drew me in, and the frozen aquarium at Wyrm's Tooth especially filled me with a profound sense of sadness at a friendship torn asunder by pride and jealousy; at something truly beautiful and great, lost forever. Rarely has a game touched me like that.
So yeah, I'll probably one of the only people sticking up for IWD. In the end, though, with GOG's pricing, does it really have to be a zero-sum either-or proposition? Go on, treat yourself: get both games. You know you want to...