Thrawn1981: The only difference between PS:T and the BG series is setting and the fact BG (and BG2) tend to be a bit more "save the world" game (although your motivation in BG at first is to just find out who's trying to kill you and why.
Punkoinyc: Pretty much, Torment has more dialogue and a less polished combat system, but otherwise it looks and plays very similarly.
Again, it comes down to
what you like in Torment.
Torment is all about dialogue, story, setting and characters, about theme and philosophy, with combat as an afterthought. Stylistically it embraces the weird, surreal and grotesque. (Beating people to death with your own severed arm, pregnant alleyways, that sort of thing.)
Baldur's Gate II is much more about the combat, and although dialogue and characters are almost equally important, there's far less weight on themes and philosophy. Stylistically it is very conventional high-fantasy hijinks, but well-executed and never feeling generic.
Icewind Dale, meanwhile, is all about combat. It has a good enough story and good setting and atmosphere, but gameplay-wise it's almost completely different. Stylistically it is far more subdued and down to earth than either of the other games, with a muted palette, understated music and a moody atmosphere.
Anyway, besides some of the titles mentioned, I'd actually recommend some Neverwinter Nights player-made modules as the closest thing you can find to the themes/philosophy/setting of Planescape Torment.
(Gameplay-wise these are different-ish. Also D&D based fantasy, but you only control a single character directly as opposed to a whole party.)
Madness and Magic is an excellent planeswalking adventure. A great story and lots of interesting places and locations to discover as you go on a quest to chase down a mysterious orb of energy that is breaching the walls between the planes. The story, meanwhile, soon turns out to be much more complicated than it first appears.
The Prophet series is a very long and meaty adventure that asks deep philosophical questions about the nature of free will, choice and predestination. It starts out in a more conventional setting (though by the end you'll reach places as esoteric as any of the Planes) but as the story progresses I found myself spending more and more time thinking about my decisions and the story. The prologue is kinda weak, though.
Elegia Eternum and
Excrucio Eternum are two short but extremely memorable stories that focus heavily on the characters and the theme of, well,
tormented characters. Although no planeswalking is involved, the setting is surreal and powerful. The second module in particular is one of my all-time favourite stories in a game.
And the best part is that you only need to buy one game to try any or all of these. :-)