Does Usability effect SEO?

How does Usability effect SEO has been a question debated for a while.

The short answer is that since Google monitors website visitor behavior and adjusts SE Rankings based on what it observes, Usability does effect SE Rankings significantly for the following reasons:

1. Bounce Rates
   A high bounce rate indicates that a visitor coming to the site following an Anchor Text-based link, has not found the kind of content they were looking for and therefore left the site quickly. Since Google is focused on presenting relevant, current and quality content in its search results for specific search phrases, if a website is not offering such content for a given phrase, Google will remove the website from its Index for that given search phrase, ergo - no SE Rankings for that phrase.
2. Short visit duration
   A short visit duration indicates that a visitor coming to the site following an Anchor Text-based link, has not found the kind of content they were looking for and therefore left the site quickly. Since Google is focused on presenting relevant, current and quality content in its search results for specific search phrases, if a website is not offering such content for a given phrase, Google will remove the website from its Index for that given search phrase, ergo - no SE Rankings for that phrase.
3. Few pages visited
   A visit of only a very few pages on a website with many more pages once again indicates that a visitor coming to the site following an Anchor Text-based link, has not found the kind of content they were looking for and therefore left the site quickly. Since Google is focused on presenting relevant, current and quality content in its search results for specific search phrases, if a website is not offering such content for a given phrase, Google will remove the website from its Index for that given search phrase, ergo - no SE Rankings for that phrase.
4. Broken Links
   Broken links on a website indicate potentially serious problems uncovered on the website's Sitemap. Since Google is focused on presenting quality content in its search results, if a website has errors, Google will likely diminish the website's position in its Index for given search phrases, ergo - no SE Rankings for that phrase.

All in all, since Google monitors visitor behavior based on Anchor-Text based visits especially, low bounce rates, multi-page and long-duration visits indicate that visitors like the content presented on the website and therefore make Google give the site more reasons to give it a high Indexing position = high SEO Rankings.