well to be honest the only thing that really needed to be tweaked (IMHO) was setting the mouselag to 0. I have to assume that they only wanted the mouselag to effect the menu's, and didn't realize it was actually in the gameplay. (i hope).
I've probably played through the demo 20+ times by now, and have found something different each time, or tried something different, or learned something that was probably obvious.
Performance wise, i've found that by setting my AA to 2X (instead of the quinqux 4x thing i usually use) i get much better frame rates, and i can't stand the "bloom" feature which just makes everything look blury. after that i can run the game in 800 X 600 acceptably. Intrestingly i don't hit any noticeable performance decline between using high, medium, or low quality models, textures, ect.. leaving me to believe that the demo only has one set, or that they don't work.
As far as the gameplay goes, i'm infatuated with it the same way i was with the first DX. There's so many different ways to aproach something, it's fun just trying to figure out which is the most fun. I mean even from the tactical point of view and you decided to kill "sak" there's a bunch of ways to do it. One of the funnier ways i've found of doing it:
Get the batton and the glass destabilizer from the first locked appartment, and KO the outside guard. Sneak up the ladder, and "borrow" the biotoxin container. Then go back down, and up the broken elevator chute, and go and talk with ms. Sak. now drop the big container of toxic gas on her desk. Shoot out the window to the heliport. Now everyone will get upset, but wont treat you as a hostile. Shoot the container till it exploads and exit via the window. Good chance that all the guards are dead along with miss sak.
The only thing i really don't like ( or miss from DX1) is the inventory system, or just the user interface in general. Click and drag is much more instinctive then the "console" style that's being used, also i loved the "space management" aspect of trying to lug around all those huge weapons. Now it's just 16 slots , each of which can hold either a candy bar, or Balistic missle doesn't matter.