OK, so after reading everyone's experiences here with 10.4, my expectations for 10.4 were low... to say the least.
Up until now, the Hive Mind's FSD opinion here on the forums pretty much matched my own, so I wasn't expecting much, if any, improvement.
Boy, was I surprised.
Just a quick recap before continuing... FSD has been nearly unusable here. I've posted before about how FSD seems to do better/worse in areas that have more/fewer testers. Elon even stated words to that effect. Anyway, FSD has been so bad here that I don't use it with anyone else in the car, as it makes everyone in my family car sick. Just coming to a stop at a traffic light is a nausea inducing experience. Not to mention the frequent dangerous crap it constantly does.
So I was more than a bit surprised when it repeatedly made acceptably smooth decelerations/stops at traffic lights. Stop signs are still a jerky mess, and not all traffic light stops were smooth, but the improvement is enough that the wife drove with me for 50 miles with FSD in use.
The follow distance bug appears to be fixed, accelerating away from a stop at a traffic light is better, handling of road cones is improved, and it no longer takes its half out of the middle on unpainted roads. It has also stopped playing with the blinker like a toy... for the most part. Also showed some improvement with turns and lane selection.
An impressive amount of progress for two weeks, IMO, but of course still far from perfect.