I won't be able to buy anything for quite a long time, but if I ever make a purchase online on GOG, I am thinking about getting this one first. Does it live up to the hype? How good exactly is this game? Truly?

[Edit] Or should I get something like Baldur's Gate 1 or Baldur's Gate 2?

I am looking for a game that is deep, can give a LOT of hours of gameplay, and can be replayed over and over again for a different experience.

I really appreciate the informative feedback! I also appreciate the fact that I am now thoroughly convinced, even though it is only 1 opinion, that Planescape: Torment is the way to go. If I ever could get it, - ... (...or WIN it one day hopefully since I am unemployed at the moment, and I just want to experience a great storyline at night while I'm not out looking for jobs during the day...) ... - I will definitely install the mods that are recommended in the mods section. Supposedly the mods can give the best Planescape: Torment experience for modern computers. :)

Perhaps I am feeling way too "ants in my pants" about the game in general, but I truly hope that I can experience this so-called treasure someday before I die... haha! :P A little bit of dark humor there. But maybe that fits the forums here in Planescape: Torment ... I hardly know much about the game, so I hope that fits anyway? The only review I have seen is Matt Barton's review on his youtube channel: blacklily8 ...

Did he spoil too much for me in his review, ya think?

If you like reading a lot (it's almost like reading a book), this is your game. The focus is in the dialogue, so if you're looking more for action, I recommend Fallout series (it was made by the same studio, I believe).
PS:T has:
Great story.
Great soundtrack (EXAMPLE).
Memorable party members (you'll miss a lot if you travel alone).
Good customization for main character (but not so good like Fallout. you can be a warrior, mage, or thieve. you always start like warrior, if you like, no need to change it. some people recommend playing as a mage, to be able to enjoy the game to the max [with high wisdom you have more options in dialogue] ).
Weak combat (please don't expect something complex, or you'll gonna be disappointed).
Bad graphics (but, c'mon! this doesn't matter >:( ) < you can improve this using high resolution patch, if you must.
Unless you play all day long, this game will take a good while to finish. If I had to give a number to replay value (1-10 stars) I would give: 4.

Baldur's Gate is good, too. I can't say much, because I haven't finished yet (I got stuck. this game is hard, sometimes).

For replay value and more diverse gameplay I'd say you should go with Baldur's Gate 2. It really is up to what sort of entertainment you are looking for. PS: T is more like a visual novel/adventure game with RPG elements and boring combat. Whether it lives up to the hype or not is debatable, I personally wasn't that impressed by it when I first played it some years ago, but the writing in the game does have it's moments.

In a bit more detail, BG2 is more like a "classical" RPG, you can select from a lot of different classes to play, with 3 sub-classes for most of them, and then there are the dual-classing and multiclassing options as well. There are tons of side-quests to complete and little secrets to find, and you also you get some choices to make along these quests, although most of them won't have a really significant effect on the game world/outcome of the story. You will also be able to acquire a stronghold for your character, which will also have it's own side-quest. And finally, you will have a high number of NPCs who can fill up the empty spaces in your party, and most of them also come with their own personal quests or chain of quests, and, depending on their alignment and personality may leave you/attack you or each other in case of a disagreement. The story is alright, doesn't really revolve around saving the world and all that typical RPG jazz, although I wouldn't call it that special either. And don't play BG1, as Whitecroc said it didn't really age well and will only put you off the whole thing. Just read a summary of the story on the internet so that you will be familiar with the premise.
NOTE: If you're not familiar with AD&D rules, you might find the battles in this game very hard in the beginning.

As for PS:T, as I've said it is more like a visual novel and less like a classic RPG. You will only have 3 basic classes to chose from (and you have to start out as a fighter), however, the most rewarding way is to play PS:T with a character who has high INT and WIS, and perhaps high CHA. Otherwise you might miss out on additional content that will help you understand the story more, which I personally find very restrictive from a gameplay/replay value perspective. This game has a lot less joinable NPCs, but since they are more tied in with the main story than the ones in BG2, they have more content, background story, dialogues and whatnot. The choices you make don't have a significant effect on the game world/outcome of the story in this game either, but you have more options to interact in dialogues based on your character's stats. Since this game admittedly puts the emphasis on the narrative and the dialogues, the combat is boring and mostly just unnecessary nuisance. The story is interesting, and while I don't find it as awe-inspiring as some other fans of the game, it certainly is different than what you would usually find in an RPG.

The only thing that gives me pause in recommending Torment is the replayability criterion: you can have a different experience playing it for a second time and making different choices, but the major revelations will feel familiar, and mechanics-wise it doesn't put much emphasis on building any particular kind of character. It may be worth looking at what people say about Arcanum, especially the kind who'll decide to play through as a female half-ogre idiot savant specialising in explosives and throwing, just because.

Torment has a deeper narrative, though, and much more developed interaction with your party. It's the surest bet if you want depth.

Baldur's Gate (the original) has more breadth than depth: it's about as close as the Infinity Engine ever got to an 'open world' game. Lots of wilderness to explore and fight through, but many maps with only a few notable encounters. You can spend ages exploring and questing in the city of Baldur's Gate, though. The sequel acquired more polish, but lost some of that breadth in the process: it has a lot of depth, and still quite a lot of breadth, but less breadth than its predecessor and less narrative depth than Torment.