So I've been looking into this HTML 5 Microdata, but I'm not sure if or when it is appropriate to use. I know that if used with rating and you search a website it will pull up things like video rating and article ratings etc. But for Microdata like People or Places, is that so useful that I should start implementing it into all my websites - big and small? How big of an impact will this really have on my SEO if I start using Microdata on everything?
The three big search engines, Google, Bing and Yahoo (and more recently, Yandex), have agreed to understand 1 single microdata vocabulary. This is Schema.org, which has examples of placement.
This formats your results as Rich Snippets, the search engine results which have pictures and fivestar ratings, etc, displayed on the search result page. While this probably doesn't have a direct effect on SEO, early studies of Rich Snippets showed that click through rates were 15-30% higher.
Note that Google says it will not support Microformats for its Schema.org Rich Snippets.
If there is a microformat for the data you have on your website and you wish to optimize your website for the search engines then you should use use microformats in your websites. How much of a difference it will make in your SEO efforts will vary just like every other thing that has to do with SEO. It will depend on lots of other factors. But if you want to maximize your SEO efforts then you will want to take advantage of every opportunity you have and microformats is one of those opportunities.
I have found that adding HTML5 semantic tags and Schema.org markup helped to have a significant affect in our SERPS; however any one improvement on it's own does not make much a difference.
My view of it is drops in a bucket. If you consider all of the ways to improve your markup for search engine optimization and consider that converting your HTML tags to HTML5 semantic tags have X improvement on your SEO. And adding Schema.org has X improvement. Adding both is going to have significantly more improvement than only adding 1 because each improvement compounds the other, making them more significant.
When I began improving the markup on my site, I thoroughly incorporated numerous improvements and Marketing told me that after a month, our ranking had improved by 700%.