There is no one monolithic HTML5. HTML5 is being developed as a single large (oh so very large) document, yes, but that doesn't reflect reality(*).
That is to say, HTML5 is a collection of loosely related incremental improvements to HTML4. Some of those new features are widely implemented already. Some of them you can safely use knowing that they'll degrade usefully for old browsers. Some of them you can use as long as you add explicit fallback code. Some of them will be shutting out some browsers for the foreseeable future. Some of them will only ever have minority support. Some of them may never be implemented at all, or may yet be edited out of the standard. And many new features that are often regarded as being “HTML5” aren't in HTML5 at all, but other standards (CSS3, ECMA262-5, DOM extensions, WebSocket...).
You will have to pick and choose which the features of the New Web you want individually, judging by current and apparent future support. There's not a switch for ‘using’ vs ‘not using’ HTML5, other than merely the doctype itself (which doesn't get you any new behaviour).
*: This was a big mistake, in my opinion. It would have been better to quickly standardise the bits that were already supported by everyone—like HTML3.2 did after the HTML3.0 fiasco (XHTML2, anyone?)—and then add new features in a more modular fashion. But that's not what happened, and it's too late to do much about it now.