I've used the nvidia dabbler a bit. It responds pretty well to pressure differences, the way you move the stylus, how you use the tip etc. It's not wacom fantastic, but it's pretty good.
I don't try to use other Art app with Direct Stylus yet (I bought my Shield Tablet 1 week ago) but interested in knowing the answer... Cause I just try to upload a picture to Dabbler 2.0 and it doesn't work When I choose the Camera it works perfectly well but when choosing a picture on my tablet its going back to my previous canvas without any error message and no picture in my different layers !