I have to admit I haven't used Newton very long, but it gave me quite some trouble. The main disadvantage probably was the outdated script in it, not the physics itself. After a while though I bought A6 pro and it took me some time to get used to that too. A6 physics needs a lot of tweaking to make it just right, which can be somewhat a problem. All in all, both are actually great when you get the hang of it, but I prefer the A6 pro physics, because basically physics works with just a few lines of code there and the manual provides good examples. I have to add though, that with a bit more research of the documents that go with Newton Game Dynamics, similar results can be achieved ...
I doubt wether or not Newton is actually faster btw, I've done some testing with the same amount of entities and different physics systems and the fps difference was less than 5 frames per second, actually in favor of 3dgs's physics engine ...
Cheers