“As long as you have great content you should do well in Google. But if people are spamming or you hire a bad SEO, that can lead to unpredictable results.” –Matt Cutts, head engineer of Google webspam team
Google is always changing their algorithm on how they rank websites in search results (and they always will), but their goal has always remained the same: To provide the best results for their users. Simply put, in the eyes of Google, the best results are the websites with the best content. So if you think about what SEO really is, it’s about developing great content and letting Google know about it.
This video below stresses that point and also warns against choosing a low quality SEO firm (“You get what you pay for” with SEO services too!):