Google have metrics on how much better it makes search - at least when I was there it makes quality a lot better, but not, say, double the quality. I think in the early days of the company they thought personalisation would be a much bigger win than it was - it was big enough that it didn't make sense to turn it off or anything like that, and you can see it in action when people say their results are customised to the programming language they are most recently using. But most of the time it's not doing all that much - and the biggest component of it was basic stuff like location and language.