I found that I managed to get the frame rate a little more stable by messing around with the Shadows options and turning them down, but not fully off (I can't remember the exact settings I used, but I think it was one down from whatever the highest setting was). Other than that, like you said, nothing else seems to change the way the game performs regardless of whether everything is on Low, Medium or High... In fact, I'm hard pressed to notice any visual difference between the various settings either, and I've got a decent machine!
Like you said, thankfully the storyline and the writing is good (if not quite reaching the pinnacle of Planescape Torment), and I'm hoping to get to play some more of it again soon (too many games to play, too little time) - but I can't help feeling disappointed by it (although thankfully it is nowhere near as bad as NWN1).
Ah well - here is to hoping Dragon Age is the return to the heights of CRPG goodness that I was hoping NWN2 would be!