Actually, I think it used to be great when it was ASAS. Once they got rid of that, and allowed control while SAS was active, it just got screwed up. I had no problem turning off SAS when I wanted to adjust my course, it just took a little practice, and the occasional fireball of death, but it was solid as a rock.
It takes some practice to use, but you can use the retro marker to guide the burns. Since your engines are on the back of the rocket (generally), you need to be pointed in that general direction for slowing down. Other than that, you want the retrograde marker roughly aligned to the forward vector marker. The retrograde target marker moves away from the direction you are facing.
Once you learn how to do it, it's not too hard, and you can get pretty close to where you want to go.