Basically useless. It can't preemptively prevent cookies from being set. You have to go to the site which of course instantly sets a bunch of cookies, and only THEN you can block the site's trackers. Which means that for every site that's new to Ghostery, you'll have to go in and manually delete the dozens of cookies that got set. It's a whole LOT of work compared to what it would be if you could preemptively block all trackers.

Note that I do have global blocking and block all turned on, they just don't seem to work right.