The dialogue doesn't come close to that of Ps:T. And to make matters worse, much of it is voiceacted, so if you don't happen to fancy a particular voiceactor, chances are that character will be ruined for you.
*some spoilers for those who haven't played though the game*
That said, there are a few characters that I really enjoyed. Ammon Jerro in particular I really liked. The githzerai girl has some interesting background lore to offer about the planes but the voiceactress is rather annoying. Neeshka borders on untolerable and crosses the line a few to many times, but she has also has some rather amusing things to say now and then. Khelgar is your standard dwarf, but is pretty likeable.
I know there are many people who like Bishop, but I can't stand having him around due to his really "bratty evil" sounding voiceactor.
I don't know, I think the trouble with NWN2 is that it's just so damn uneven. There are places in the story that I genuinely very much enjoyed, even though choices&consequences are often non-existant or very small. I loved going through Jerros Haven and dealing with the demons there, I thought managing Crossroad Keep was a lot of fun etc, the trial scene was excellent. But it's "sprinkled" with braindead dungeon passages, and that especially goes for the first Act.
While the rest of the game is not close to the best I've seen, I think it was definetely decent enough to warrant a purchase.
And yes, Avellone did write parts of it (the trial scene I mentioned before). I don't know how much though.
*end spoilers*
To make matters worse for you, the worst aspect of NWN2 is its hopelessly inefficient renderer, which poses ludicrous hardware requirements while delivering poor performance and outdated visuals. Before you can even think about playing the game on your system, you will need to get more RAM and a new video card. Once you have that, you can probably expect to get somewhat acceptable performance at lowest settings (which ironically make the game look worse than original NWN).
Worse than NWN1? Hahahahaha, that's pretty funny. No game looks worse than NWN1. What's funny though is that my performance in NWN1 is almost identical to what I get in NWN2. I still get slowdowns in NWN1 in areas that have many NPCs in them, and when lots of spells go off etc. In NWN2 I have the same problem, but overall the game looks about a million times better.