Google's newly launched personalised search has attracted little fanfare outside the SEO community, however it has the potential to be significant to every internet user: The way we use search, the way we use the internet in general, and it could even have a significant impact on the fortunes of many businesses and organisations. This update is arguably the most significant to Google's search for several years, and will likely have many more repercussions than are immediately obvious. Personally, I also think there is a somewhat sinister side to the change.
In terms of improving user experience, personalised search could be a good move, if it does indeed improve the quality of results returned. But what about people who are bad at searching? Yes, for those of us that know how to come up with sensible queries that will find us the results that we are looking for, and can spot a spam result without clicking on it, we may see sites that are useful to us appear more often in search results. But what about the average, or less than average searcher? I'm sure most of us have one somewhere in our family: They surf the web through a letterbox made by a 400px high stack of toolbars and manage to repeatedly infect themselves with spyware. Potentially, they could end up with result pages full of garbage. If they are in the habit of only clicking on the first couple of returned results for queries the problem could be compounded.
We may even see malware being created which exists purely to search Google and visit links in the results, to boost the ranking of certain sites for certain queries on the host PC.
Another issue I can see is times when surfers do not want to see results they have already looked at - they want to see fresh or different things they haven't seen before, for example someone scouring message boards to try and find a solution for a technical issue. Adding an option to disable adjustment to rankings would help this.
But what really worries me is that it's hard to imagine this change not having the effect of slowing the rate of change of search results - new sites and brands might find it more difficult to rise to the top, even if they offer a superior service/product: Big names and top results could become entrenched at the top of SERPs. I bet there will be a lot of large organisations and multinationals breathing a sigh of relief with this news - as it will likely lower their chances somewhat of being felled from the top of the rankings, now or in the future.
Finally, I think Google are really onto a winner with personalised search - cookies have rarely if ever been all that valuable in the past, but imagine the situation in a couple of years where someone without a Google Account who has been building up search history buys a new computer. Suddenly their search results are completely different. What lesson they might learn, if they are not happy about the change? Get a Google account.