I've used it since it was Junkbuster. It always did well enough that I didn't see the need to change. Will look at the two you mentioned.
[edit]
Just looked at adblock. One thing privoxy has is you can keep your rules up-to-date with the list they maintain. Decentralizes the arms race with the ad merchants.
It also handles header re-writing, javascript and cookie controls, lots of stuff besides just the images.