Ghostery, AdBlock Plus, et al - how effective are they REALLY?

[Naimfan]

As the title says - how effective are extensions/add-ons like Ghostery, AdBlock Plus, and the like at preventing other sites from tracking someone?

Moderators - if this is more appropriate in a different section please move it - seems like it would fit in several different areas. TIA!

 

[sergeyz]

ABP blocked "1,589,268 in total" since i added it as a Chrome extension. Once I turned it off and went back to the world of advertising. It was crazy. However, there are some sites on my "white list".