I'm almost done with A knight of the Seven Kingdoms. Pretty good although it lacks a certain spark that ASOIAF has. I would rate it a 4-4.5 out of 5.
Dunk & Egg are the most traditional of the Game of Thrones characters and imho GRRM could have kept them in the main series. Dunk would have been a fan favorite, an alternative for Jon Snow but he would be too similar to Brienne of Tarth.
Overall the novellas are good quality, very well written but fairly conventional in storytelling/tropes, unlike ASOIAF. Very competent but not that memorable.
I think the new HBO show only has 1 season's worth of material and the entire book will be covered.