Seamless installation and execution. I've used Ghostery for 90 days or so with no issues, it's worked on every site and browser launch. The user is in full control of what the add-on tracks/blocks/reads/stores/etc. Of course the ad blocking is nice, and the personal privacy is paramount, but my favorite part is how much faster Firefox runs with Ghostery cutting out the prefetching and predicting that standard browsers seem to be so obsessed with. If performance optimization is the only reason for your download, it's reason enough.