This subject is really in the news at the moment. I recently wrote a blog post about the extent to which you could trust search engine results, and was it morally or ethically acceptable for them to choose what results to give enquirers. Legal scholar Tim Wu, Harvard Business School professor Michael Luca and data scientists at Yelp have produced a paper that is highly critical of Google and the results that it returns. They claim that when people do a local search Google is returning results which are favourable to companies that have an association with Google. This means that if you search for 'plumber' you're more likely to get plumbers who have Google+ pages in the list of the seven local results that are returned. This is a set of results from the 'Local OneBox' in which Google gives you its top matches for your query.
As you can see, each and every one of the results has some relationship with Google already, either a G+ page or reviews. “The fact that Google’s own algorithm would provide better results suggests that Google is making a strategic choice to display their own content, rather than choosing results that consumers would prefer,” the paper claims. Google of course isn't at all happy with this, and given that Yelp is a direct competitor, they've every right to say that. However, I'd suggest that you make up your own mind; when doing a local search don't just click on what you can see, delve a little deeper.
Another article, from Ghacks is reporting that Google has penalised the website Kickass Torrents, a leading website. Worse, the article states that Google is instead returning an imposter website that contains malware. Take a look at the results from Google and DuckDuckGo:
There's a very clear difference between the results, and the ones from DDG are far more helpful and transparent. Now, you could argue that Google thinks that there's something wrong with the site and that it's being penalised for some transgression or the other. However, since DDG and Bing are emphasising the official site in their results I have to wonder what Google is seeing that no-one else is. Furthermore, after going through several pages of results I was still unable to find the official site at KAT.cr If you do a site:kat.cr search results do come up, so it's not that Google isn't indexing and and linking to it, they've just decided for some reason that they don't like it.
Now, if you're looking for material and it's not an area that you're familiar with, you do a Google search, and you're pretty much going to trust the results that you get. After all, what else can you do, right? Unfortunately I think we have to be even more on our guard these days, and remember the mantra 'don't trust a search engine, ask a librarian instead'. Certainly double check results on anything that's important with another search engine.