40 to 100 Hours
The Bottom Line:
I played the original NWN and was sorely disappointed. I could never get past the first chapter, because I found the whole thing exceedingly boring, and I got lost from sheer lack of directions in the expansion packs. NWN2 fixed that, in several ways.
The first is with the characters you encounter. The people actually seem more real, and you can build relationships with your companions based on your actions. The characters have more depth to them, and have backstory and quests to go with them.
The second is the plot. Apparently, it's quite typical of adventure games, but I wouldn't really know. I enjoyed the plot, and unraveling the mystery. Parts of the first chapter were a little tedious, but were still fun (at least on the first play through). This is particularly true of MotB, which has a truly epic plot and even better characters.
Third, you can now control all of the player characters. Instead of only being able to give suggestions, you can equip them the way you wish and tell them exactly what to do. I've found that by tweaking the AI slightly, you can get a party that doesn't need turn by turn guidance, but figuring out just what you want can take a while.
Playback value varies, because it depends on how thoroughly you did everything the first time. There are two sides to choose from in the beginning, so I suppose you could play through it twice to see how each one plays out, but other than going back and finding little things, there's not a whole lot to get the second time around. If you enjoy playing with different characters, however, and don't mind the same plot over again, it is a wonderful replay.
Challenge will vary depending on the person and how well you level. For NWN2, I played the first time and missed several side quests, and was about five levels below recommended. This made playing hell, even on the easiest level. On the other hand, if you do it right, you can blow right through most of it.
There are a few cons to the NWN2 as well. You are "herded" a lot more. There is less space to wreak havoc than in the original. There's much more guidance, and you have to follow along the main plot or not continue with the game. There are enough side quests and different things you can do that it doesn't really matter, and you still get to make choices, but the basic plot and events will stay the same regardless. I rather liked this, because it means you don't get lost wondering what to do nearly as often. Some of the quests are rather annoying, and quite a bit tedious, but only one or two stand out in particular. I didn't notice the problems with the camera in NWN2, but I had a hard time adjusting to the change in MotB. However, it is possible to tweak the camera until you find what you're comfortable with.
I patched mine to latest version. This RPG comes out with patches frequently.