OK, this is an initial impression, so it may improve when I get used to it, but I hate it, almost to the point where I don't want to play the game any more.
Haven't tried it with rockets yet, but it's making my spaceplanes much harder to fly. Firstly, the claim that it allows you to adjust one axis while keeping the other 2 stable seems to be false. I'm getting phantom rolls when I yaw and pitch, phantom pitches when I do the other two, and so-on.
Secondly, the "lock to heading" functionality is completely gone. The thing will drift, regardless. The old ASAS could be used to pick an angle of attack and hold it, so you could build up speed while in straight and level flight, just below your flameout altitude. That ability is gone now. They drift up and down, wandering about by themselves, so while trying to pick up speed I'm getting random flameouts which I could avoid before. I'd say this is taking 100-200 m/s off my top speed on jets, which when you're trying to save rocket fuel is a *lot*.
Thirdly, even in space it seems it will only hold a heading if your plane is perfectly balanced. It turns out mine has a very slight off centre thrust on its rockets. The old ASAS simply used the pod torque and gimbals to counter this. The new SAS doesn't hold a heading, so the moment I throttle up the direction starts to change and I have to make constant manual corrections. Burning for a manoeuvre node seems to be more or less impossible.
I do appreciate the removal of the random oscillations, but please can we have some sort of heading lock functionality back?
ETA: Some more flying in space. I'm getting phantom rolls. I can understand a slight unbalance causing phantom pitch, but the only thing that's possibly off centre in the roll axis is the fact that I have a ladder on one side. As far as I can tell, SAS is making no effort to counter this at all.