Many people seem to think customised search will make SEO less effective. However I don't think it will make much difference due to the below reasons. Would be interested to read other people's opinions. 1. Most searches are for things people haven't searched for before so there will be no search history. 2. If they have searched before and been on one site several times it will only put your site down one place compared to normal. Plus they will already be aware of this site and either visit no matter where it is or ignore it because it doesn't provide what they want. 3. Google has said a maximum of 2 sites will be on the first page due to this, and never the top ranking. So effectively top 8 is top 10 and position 1 will still be position 1. In a way it will make SEO more important as you will need to make sure you are found first time people visit and then take advantage of customised search.
I think you mean personalized search. Google's custom search (http://www.google.com/cse/) is something completely different. What's the evidence for this? It isn't true in my case, but then, I am an SEO. Others' mileage may vary. But I think a goodly number of people use search in lieu of bookmarks to return to this or that site. Bookmarks can be unwieldy by comparison. Valid point. Could you please provide the the source of this statement? I have definitely and undboutedly seen personalized search put a site in position 1. Do we really know that vising a site once advances it in personalized search? I would expect it to take more than this. Google looks at user behavior, so if that first visit was a bounce...
Personalized search does not make a different, especially when Google did remove them from default option when people do search. I have experienced for both "Personalized Search" when did log in some of Google services and "Web History Based Search" when did not log in any of Google Services. That was annoying me too much. Those 2 things help me nothing but deliver a result that do not matched for my query.