I keep hearing great stuff about Planescape: Torment. But I've had this problem before, so I gotta ask. Does this game's greatness run on nostalgia?

I wouldn't say so. I played it for the first time not very long ago.
Personally, I don't like the game because of the god-awful Infinity engine, but if that's not an issue for you - let it rip!

Not sure I've ever heard anything about the Infinity engine, what's exactly wrong with it?

I've been playing through it for the last month, and I don't think it's greatness is nostalgia alone. I personally think it would have been more suited as an adventure game and that the D&D trappings occasionally limit it, but it really is one of the best stories in PC games.

I highly recommend it if you're into extremely unique, bizarre and compelling stories, but definitely check out the modding guide GoG posted to get it running perfectly.

EDIT: I wanted to clarify that this last month has been my first time playing it. I hadn't even heard of it before I joined GoG. So I am completely nostalgia free.

'Sacrifice' is a good first person strategy / RPG.

I simply dislike it, but I'm pretty much alone in that regard.

First play Anachronox, after that Anachronox and finish with Anachronox. After that you might try playing some... Anachronox. Seriously, it's one of the best western RPGs.
As for Planescape: Torment, I don't think its greatness has much to do with nostalgia. I have played it not so long ago for the first time and it was awesome.
When you say you played most of the well known RPGs I assume that you have played:
1. Deus Ex series
2. VtM: Bloodlines
3. Fallout series
4. The Witcher duology
5. Final Fantasy 6, 7, 8, 9
6. Anachronox
7. Parasite Eve series
8. System Shock series
9. Penny Arcade Adventure duology
10. Baldur's Gate duology and Planescape: Torment
11. Xenogears
12. Lunar duology
13. Chrono Trigger & Chrono Cross
For really obscure stuff:
14. Monster Girl Quest
15. Utawarerumono
16. Tears to Tiara

Have played Deus Ex, Fallout 2, Penny Arcade, Baldur's Gate aaaand Bloodlines.

I'll try the other ones you've suggested, thanks for the list. Though I imagine some of them are console games? And Witcher is too new for my PC.

You should know that System Shock series are the most requested games on this site. If you get the games through the other means, know that it might be trouble to launch them on new machines, however I managed to play and fully finish SS and SS2 on WInXP. It took some work to get them to play correctly though.

It is said that some people also have trouble playing Anachronox, but I had no trouble to play it on my new machine. And it costs only a dime on amazon.co.uk

Other games I mentioned are for PlayStation1, but I have played and finished them on an emulator without a hitch. Final Fantasy 7 and 8 have PC versions too, but I strongly suggest getting PS ones due to stability, compatibility, size and savestates.

About obscure stuff. These three are all PC games.
My review of Monster Girl Quest is here. it will tell you what kind of game it is.
On the other hand Utawarerumono is probably my favourite game of all time. It's a VN/Tactical-RPG hybrid with wonderful story.
Tears to Tiara is from the makers of Utawarerumono, but seems like a downgraded version of that game. Personally didn't like it that much, but I know some people that preferred it to UTW.

Speaking of dangers of buying alone...
What the F is this?
http://www.gamersgate.com/DD-DETER/determinance
It get it, that it's by the guys who made Frozen Synapse while high after playing Die by the Sword. But.. why?

Maybe it has an appeal of some kind... No demo, though, don't think I'll spend 6 euros to try it out.

There's a demo at the game's website http://www.determinance.com/

Tried the demo, and I must say, it sure has charm.

Guys. Sequence has a sweet promo right now. Anyone play it? Any good?