The final release of HTML5 will be like another decade away. While it is so far away, it doesn't mean we can't start using things from the spec now.
Anthony.Barnes posted a link to [Can I Use, which is a great guide to things that you can start using NOW that are supported in many browsers. For browsers that don't support a particular feature there are many [URL="http://remysharp.com/2010/10/08/what-is-a-polyfill/"]polyfill](http://caniuse.com) scripts available.
HTML will likely end up somewhat a "living standard", which would mean the language (in theory) would always keep evolving.
The challenge is of course for browser vendors to make sure that their browser is backwards compatible with any changes in the spec.
While there are certain elements of HTML5 that probably aren't quite ready yet, there are many things that are. And as the spec evolves and matures, and browsers continue to update, it will definitely be one of the better ways to build websites IMO.
Having said all that, nobody has to start using HTML5, XHTML is still a valid standard, as is HTML4. As with most things web dev related, there are advantages and disadvantages to using particular technologies. It's up to us (developers / IT folks) to weigh those up and make a decision about which technologies we will use to build our web stuff. (Web stuff is a technical term, clearly;))