I wrote this post a while back. The content can still be relevant but the information I've linked to may not be available.
Oct 20, 2006 by Clive Walkerevent
Yesterday, I atttended the London WSG Meeting - Microformats. Here's my quick review.
Next, Jeremy Keith described his own experiences with Microformats on his own website. One lesson I learnt was that you can use Microformats as little (or as much) as you like. There's no absolute requirement to add additional content to your website in order to become 'microformatted' (unless this is required for the microformat). You can add more if you want but, in a lot of cases, it's just a matter of marking up the existing content.
Thirdly, Drew McLellan gave perhaps the most interesting presentation by asking the question, "Can your website be your API?". It was interesting to see websites API's compared with the use of Microformats in order to provide greater access to website content. In particular, Drew M highlighted the Corkd website, which does not have an API, but uses microfomatted data to a high degree, thus providing its data for use by other websites. In a lot of cases, microformatted content might be a substitute for an API? In truth, the answer to the original question was "No, but Yeah, but No, but....". In some cases the answer is definitely 'Yes'.
The meeting featured three excellent speakers and I know a lot more about Microformats as a result. In particular, the hReview Microformat  looks like something I can use on a regular basis. I can see how implementing Microformats will lead to a greater use of current website content/information. From a purely personal perspective, and putting aside any altruistic motives, the use of Microformats can bring greater value to a website, providing increased website exposure - and lead to a greater number of visitors.
1 I have attempted to create this post with the hReview microformat. Mainly to see what is involved. Please excuse the duplicated date at the top of the post
Further reading: Microformats cheat sheet