I've played a lot of computer based role playing games and I think Neverwinter Nights is the best. I've played it for over a year now.
For one thing, the Platinum package offers an incredible amount of content that used to cost $100 in a very affordable package. Second, the game is based on Dungeons and Dragons rules, which have been tested and refined for decades. It takes a while to grasp the richness of the possibilities, but it's worth it. Third, some people complain about the graphics but I think they look great. I never get tired of pausing a battle and zooming the camera around in circles and in for a closeup. Fourth, the interface is so simple it's almost a work of art. You right click on any particular object and a menu of things you can do with it comes up.
Fifth, with NWN comes a toolset that lets you build your own adventures. The ones provided with NWN Platinum are pretty good -- with Shadows of Undrentide and Hordes of the Underdark being far better than the original campaign. So you can download, for free, literally hundreds of additional games that fans have spent hundreds or thousands of hours creating. And many of these are even better than the adventures that come with NWN Platinum. People have also built additional scenery, monsters, environments, and much more. So you learn one set of rules and you're set for dozens of adventures - some short and intense, some huge and sprawling (with up to 60-100 hours of playing time, so they say). I feel like I'm totally set to be entertained for the next 6-12 months, probably more if people keep building great adventures and putting them online. (See nwvault.ign.com for a glimpse of what's out there. And did I mention it's all free?)
Sixth, if you have a fast online connection (which I don't), you can play online with others in "persistent worlds" full of gamers, or in campaigns with your friends -- and a Dungeon Master can even run the show and provide the kind of interactivity and richness that pen & paper experiences used to provide. I can't speak to that whole aspect because I don't have DSL or cable, but many people love playing that way.
I think this is simply an unbeatable package. Diablo I&II were incredibly fun, they had atmosphere and gameplay for days, but in terms of role playing they lacked depth and freedom, and battles were basically a clickfest (click on that one! now click on that one! now click on that one!). Morrowind is unmatched as a sprawling, freeform adventure, but it's so open ended that you often feel lost and somewhat unmotivated after a couple of weeks, or months, of playing. Plus, to improve your lock picking skill, you had to pick hundreds of locks. To improve your jumping skill, you had to jump and jump and jump. That system just doesn't appeal to me.
I wish you good gaming. [PS: advice for new players: play a cleric. Great combination of fighter and spell caster. Lots of fun.]