There are 4 reasons to play the Neverwinter Nights games.
1- The D&D 3.5 edition ruleset
(NWN1 uses 3rd edition, which is also excellent) and it's absolutely mind-boggling array of character options. There are so many ways to create and play a character that if you're only experience with rpg is through a game like Dragon Age you're mind might quite literally explode. This is a game that lets you build and play a character how *you* want to do it. 3.5 is also my favourite D&D ruleset to date. 4th edition can suck it!
2- The multiplayer is exceptional and one of the few rpg out there which not only supports multiplayer but DM's as well, meaning you can play entire mini-D&D campaigns from the comfort of your home complete with a human guiding the adventure and without the need for dice rolling or peripheries. There's also 'persistent world' servers which act as mini-MMO's, many of which are roleplaying-enforced if you're into that sort of thing.
3- The mods and fan-campaign. There are, I kid you not, hundreds of these out there. Many of which are of such high quality they're free expansions unto themselves. You wanna play a story where you're a ghost seeking revenge for your death?
Revenant. How about a campaign where you get to play as an evil Drow within Drow society?
A Hunt Through The Dark. Maybe you're sick of Forgotten Realms as a setting and want some hot Ravenloft action?
Misery Stone. Hell, wanna play a semi-pornographic R rated adventure where you're a princess on the run from an evil empire taken under the wing of a thieves guild for protection?
They've got that too and it's actually pretty damned good!
There are so bloody many campaigns out there, many of which are of exceptional quality, that it's absolutely mind-boggling. I've been into the whole NWN1&2 modding scene from the beginning and even so I've not even played half the stuff that's out there. There's just too much, and it's
AWESOME. Buying NWN 1 and 2 is like opening the gateway into an entire world of rpg possibilities.
4- Mask of the Betrayer, NWN2's first expansion pack and easily the high-point in the entire NWN series in terms of developer content. Excellent story, fun setting, memorable characters, surprisingly beautiful visuals, mature premise, and an excellent soundtrack all combining to make it one of the best rpg experience in years. For the original NWN Hordes of the Underdark, it's second expansion, was pretty good as well but nowhere near as memorable as MotB turned out to be. NWN2's original campaign was alright, but it was held back by the fact it took absolutely forever to get moving and a disappointing ending. The story doesn't really pick up till after you've invested hours and hours into a tedious first Act or two, and I don't blame many people who quit before ever getting past that section. It's boring.
Mask of the Betrayer makes it worthwhile however.
And in answer to your four questions, from what I've heard it works fine on Windows 7. The game was indeed released with some severe issues, but for the most part that's been fixed up over the course of two expansions and countless patches. NWN2 looks pretty good actually, I'd say on-par with Dragon Age: Origins if not slightly better simply because the colours tend to be a bit more vibrant. Nothing will blow your mind visually, but it's not so bad looking as to push most people away. NWN1 is a bit of a different story. And while it might be a bit hard to get into initially, it's not so complex that I think you'll throw it out the window. I know a couple people who didn't know a thing about D&D and enjoyed the NWN games regardless.
