I used Ghostery for a while but found it redundant and onerous since I also run NoScript. I agree with the parent that said Ghostery was a poor man's NoScript, though Ghostery only blocks known trackers (analytics tools) and will let random evil scripts through. NoScript allows you to whitelist script sources by domain, so you usually see "Enable example.com; Enable analytics.google.com; Enable quantcast.com;", etc., and you can just enable example.com if you don't like the trackers.

NoScript is really important and useful I think. Less convinced on Ghostery. AdBlock is useful but I don't know if I'd call it "important" really; it provides a great visual convenience to be able to read sites without being bombarded by ads, but one can live safely without it if necessary.

However, NoScript is still very useful for (imho) two things: disabling extra functionality that you don't use in exchange for faster loading time, and for browsing on potentially dangerous websites.