The fact that there isn't a dedicated thread for this monumental game is beyond me.
Let's hop back in time a bit and see what people thought about this game in 1999:
PC Gamer
With so many different ways to get through the game, it's likely that many players will go back through just to see the alternate solutions and discover some of the secrets they may have missed. When it comes right down to it, this game is a masterpiece of roleplaying—the dialogue is some of the most well-written of any RPG, the environments are varied and downright enthralling, the spell effects are mindblowing, and the story contains some of the most inventive, unique characters players will ever have in a party. Whether or not you're a hardcore fan of RPGs, this Torment is a must-have.
PC World
GamespotI have never before seen an RPG so deep, so entertaining, and so damn good.
It's clearly the best traditional computer role-playing game of the year and is bound to be an all-time favorite for many of its inevitable fans. That's because it's a great-looking game that's lengthy but never boring, and it begs to be played through more than once, just as its unique hero and his story promise not to be forgotten.
ActionTrip
There is not even one aspect of it deserving disapproval, and it is with extreme partiality that I claim that it is one of the best RPG games ever made.
IGN
It's got enough depth to keep you entertained for a VERY long time..., and perhaps most importantly, it's a hell of a lot different than anything else that's ever been released.
This game in my humble opinion tells the best story ever told in the medium.It's pure interactive literature with deep rpg elements and an damn amazing setting.If there is a remaster i beg to see,it's this one....
What can change the nature of man?
Useful links
Why you should play Planescape:Torment
Planescape:Torment story review by Giantbomb
Planescape: Torment Main Theme
Mod Guide