As others (may)have mentioned already, personalization is a tricky business. You may get more false positives ( results that are not quite what you were looking for ), which can be okay but you may even get the kind of false negatives that could lead to perfectly suited to your query pages falling way down in the results set.
You may even have to worry about what you are looking for, worrying whether it may confuse the system (e.g searching for a given subject may affect future results based on the fact that you searched for that said subject, even if you may no longer care for it, once you are done researching ).
I am sure those and other effects can and will be minimized but it seems unlikely this could become a 'solved problem' anytime soon.