Thanks. So it is worth to use HTML5, isn't it?
To the extent that it's cross-browser compatible, sure. But with IE 6, 7 and 8 in the mix (and to some extent, IE 9 as well), that doesn't leave you with much. HTML 5's biggest contribution has been the <video> tag, and even that's used almost exclusively for the iPad, since it doesn't support Flash.
As MarPlo pointed out, you can still use XHTML syntax. Though, those extra slashes don't come with any benefit. And yes, with some JS and CSS magic, you can make IE recognize the newer HTML 5 elements. But I haven't heard anything from Google saying that <article> is more SEO friendly than <div class="article">. So, again, not a whole lot of benefit.