I have tested, numerous times and over a period of years, and I get very different results when I'm logged in and when I'm not logged in. I have the option enabled to track me and personalize my results. I have checked the differences with use of alternative browsers and devices, while using a VPN and not, and even just using alternative profiles.

I also have a media box (well, a series of them over the years) that uses an entirely separate profile. That profile does a lot of YouTube and music. It had learned that I prefer documentaries - though not much more than that. It has also seemed to learn that I prefer isolated guitar tracks and classical guitar. It will now preferentially offer me the isolated guitar track in the top few links, even without including it in my search query. It used to offer different versions, covers, and live versions of the track. I can't get it to offer the isolated track as the primary result, at least not at this time.

I've been monitoring this for quite a while and have watched the improvements. I've even set up the second account, the one mentioned for media, just because I'd observed the effect.

To see the effect, you need to be logged in AND tell them to personalize your results. I'm not sure if that is enabled by default.

I admit this is just anecdotal, but I've made it a point to monitor this since about 2010, which is when I first noticed the option in the settings. I've checked it against being logged out, with different IP addresses, with alternative browsers, and on computers that do not belong to me. The results are remarkably different, though they are pretty generic after the first 20 listed results.

So, in this case, I've given information (privacy) in exchange for results that are more suitable for my personal needs.

I do wish I could select profiles. I'd like to be able to search with my 'generic' profile, my 'tech' profile, my 'music' profile, and my 'entertainment' profile. I think it'd give me more precision and would help me weed out my more generic searches.

Unless you have it blocked, Google also uses a tracking URL on their results. It will say, even if you mouse over it, example.com. But, when you click on it, it actually uses a URL like google.com/tracking/some numbers/example.com. So, they know what links you clicked. I suspect they also log which one you clicked last, so they have an indicator as to which result you found the most helpful.

I should note that links open in a new tab automatically, so they can also track browser session and see when I've completed my searching and can infer more from that information.

I used to also have a developer profile but I haven't really done any development for a few years. That was pretty refined and seemed to learn the languages that I was most interested in.

In each case, the profiles were made by using different email addresses. I have a different email address for the different accounts. I try to avoid cross-pollination between them but I haven't been as good with that as I'd like to have been. I can only speculate that the results would be even more precise if I had done so. As I mentioned above, I did use a third email account but I've not logged into that one in quite a while. It seemed to pick up that I was into PHP, JavaScript, HTML, and was using Wordpress or SMF. I don't know how effective it would be with other languages.

Your mileage might vary, but that sums up my observations and experiences. In my case, I made a clear choice to give up some privacy in exchange for something I value - namely more efficient search results that are more effective and personal.

Again, all an anecdote but I have made it a point to test the results against non-personalized results and the difference is very noticeable and very good. I am much, much happier with the personalized results.

I slam on Google, a lot. I complain about Google - to the point where people may think I hate them and am obsessed with telling people. However, I do try to always make it clear that I'm okay with them asking for my information and that they do have some products and services that I think are done exceptionally well.