I have never played it, but it's like $3(plus another $4 for shipping) on amazon and I am tempted. I've read mixed reviews on the single player (both from "professional" review sites and user reviews)
I trust you guys, so let me put it this way in a hypothetical. If Neverwinter Nights was avaible here for $7 and it was single player only, would you buy it?

I thought it was enjoyable enough, though, others might not think so. You should also get the expansions, because the campaigns are far superior, and for the mod support. The greatest thing about the original Neverwinter Nights is the awesome mod community, which has made many awesome singleplayer and multiplayer campaigns. So, yes. It's worth it.

The NWN1 campaign is kinda crap but not so bad that I regretted buying it at full price when it first came out. The problems are basically that it feels a bit empty in places and it feels like you can't really have enough of an effect on the world in a few instances. For 7 bucks you'd be mad to not get it.
However, if you can get the diamond edition or whatever one comes with shadows of undrentide and hordes of the underdark, I'd go for that since those 2 improved the gameplay by a fair bit (HotU being amazingly cool)

I just bought the Diamonds version for $11.49 total with shipping.
I sure am easily persuaded

Well for that price its certainly a good buy. Even though the undrentide & underdark campaigns are generallly better (and don't use the same character as the main campaign), you get a lot more out of the expansions if you know the chracters and events of the main campaign so don't just skip that in favour of the better ones.
I'd definitely recommend a fighter as your main character though, I have serious doubts about the survivability of mages

NWN1 is basically a Diablo clone pretending it's a proper D&D game. Just remember that and ignore all the worthless non-combat classes and skills and you should be fine.

The NWN1 single player campaign (original campaign) isn't that good. It's very linear. Each act is typically based around a hub from which you can go in three directions to complete three main objectives. There are a few sidequests thrown in - and I do mean a few.
It's certainly worth $3 though.
The HoTU expansion was the high point of NWN1 single player, really.
The multiplayer, though, is still alive, kicking, and bucketloads of fun.
Oh and congratulations on the diamond version. Does that include the premium modules as well?

You've already bought it, but I agree with the others - the original campaign is weak, but fairly inoffensive, while Shadows is pretty decent and Hordes was really good. I don't really agree that you need to play all three in order, though. There are some connections between the campaigns, but with one notable exception, I can't really think of any strong ones. You might as well try the OC first to see if you like it, though.
However, you should really check out the player-made campaigns at the Neverwinter Vault. There are some really great adventures out there. The only downside is that combat in NWN is serviceable at best.

Another another agreement. The first OC is rather poor (passable, but nothing special or really memorable), the expansions better and better. You can port over your character from the first campaign to Shadows and HotU, but you'll be rather overpowered starting off.
A lot of fun comes from multiplayer and the community campaigns, a huge amount of extra content there.
At that cost, go for it.

Well I know I liked it, not as much as HotU but probably about the same as much as I liked SoU.
Largely for Linu & Aribeth, notably the latter

I'm surprised you liked any of it then. As all HotU did was stop pretending that NWN is anything more than a Diablo clone.

My personal order of preference (highest to lowest) is Hordes, original campaign, and Undrentide. For some reason Undrentide just felt a little underdone to me, even as a short add-on. The elements just never quite gelled.
Hordes, however, could pretty much have been sold by BioWare as a true sequel. Undermountain's a place where rogue PCs can really feel useful (points in search are worth a lot of good stuff and at times disable traps will make life much easier) and I appreciate the integration of puzzles of the sort found in pen-and-paper campaigns.

I got NWN1 and the 2 expansions.
Although I install/reinstall NWN1 quite regularly, I never get much past tutorial and Act1...health and RL-time issues, rather than crappy game.
Never got around to installing the expansions.
Gameplay was pretty good. Graphics were excellent. Story....acceptable, but not great.
For that price, grab it.

NWN's main campaign is pretty good, I think, primarily because it educates you in just about every facet of Dungeons and Dragons gameplay. Mods vary from absolute crap to absolute gold, but almost none of them will give you the variety of experiences that the main campaign will. The story's not bad, and the gameplay's not bad either. The only real problem with it is that it can be fairly difficult if you want to play solo on harder difficulty modes (1st-level wizard = dead man walking), and since it's mostly linear there's not much replayability to it.
Shadows of Undrentide is even more linear, which can actually be kind of funny if you intentionally play a sociopathic character who treats everyone like crap but is expected to save the world anyway. Again, it's not bad, but not as good as the main campaign.
Hordes of the Underdark, being a more-or-less direct sequel to SoU, is more of the same at a higher level. The story is a tad more epic though, and it's significantly less linear in that you can get multiple endings depending on what choices you make at key points in the game.