The idea of personalisation is that if a user is, for example, a keen cook, searches linking to a particular recipe website are more likely to end up at the top of a user's search results, sometimes overriding extreme search engine optimisation efforts by publishers to get to the top of the Google search rankings in the process.
Understandably, this has ruffled feathers at search marketing agencies.
One such agency, Greenlight, said the introduction was a threat to our way of life.
Greenlight chief operating officer Andreas Pouros said: "This might restrict the breadth of sites that are delivered to the user, leading searchers to only see sources of information that they typically agree with, which deals with cognitive dissonance in a detrimental way for society at large.
"Consider how this might affect search results for Barack Obama if he searched for 'health care reform' or Al Gore if he searched for 'climate change'."
The new service has been made the default setting in 40 languages and to users all over the world.
Prior to this launch, personalised searches are available to Google account holders when they are signed in and have checked an option to save their web browsing history.
Mindful of privacy concerns, Google has created a video explaining how the service works and how users can opt out if they wish.