Privacy
Like many of Google's services, there's a trade-off involved.
Those using the service will end up disclosing ever increasing amounts of information about their interests and habits.
That helps Google target its ads, but also opens it up to criticism.
"They're gobbling up everything they can learn about you and trying to monetise it," said Apple's chief executive Tim Cook earlier this year.
"We think that's wrong… and we think some day, customers will see this for what it is."
Apple has made much of the fact it anonymises information gathered about users, meaning, for example, it sends location-specific alerts without recording their travel histories alongside their account IDs on its servers.