There are some things you need to know about Neverwinter Nights 2.
It's somewhat buggy (the expansions much less so).
There are long load times.
The game does not run that well on older computers (or fast computers from during its time of release).
The combat really kinda just sucks - not even a difficulty problem and more just the way the engine handles everything.
It can be crash-prone.
The original campaign is horribly generic and features way, way too much filler. It's badly paced, boring, and while it has some bright spots (the trial sequence in act 2 is awesome, and upgrading Crossroad Keep is fun), you may well find yourself growing tired of it very, very quickly.
If you are willing to put up with that, then you will find literally hundreds of hours of solid to excellent RPG gameplay, with fairly nice graphics and an excellent soundtrack. It is very "old school" in terms of the way it plays, which is a style that's in short supply for a lot of RPG fans these days.
Additionally, the expansion packs are all excellent. Mask of the Betrayer has the best story/world/choice & consequence to it of any game Obsidian has made since KotOR II, and feels very Planescape-like, while Storm of Zehir captures the tabletop feel with very heavily mechanics-driven play - Obsidian managed to make all those D&D skills and feats actually useful. Story in Zehir is not really interesting and arguably it's a bit of a "failed experiment", but it's still worth playing.
I haven't played Mysteries of Westgate just yet, however, I have it on good authority that it is a very good RPG, and highly underrated.