You'll find some good Adobe TV videos on HTML 5.
It is a very good thing and works well for most modern browsers as it avoids the tangle of plug-ins and can draw objects in ways that previous levels could not.
The integration implementation however is a pain in the ass. You have to code exceptions for every browser none work with the same tags. Incredible after all these years and this still goes on.
You also then need to make them compatible with older browsers too, yet that shouldn't pose too many problems, you'll just loose the effects.
Well, it isn't a standard yet, so there is no reason why it's features would be nailed down 100% yet. That said, the bulk of your standard use things are either implemented mostly the same, or can be gracefully degraded.
I have started building all the websites I work on in HTML5. It's a bit confusing getting the semantics (Aside vs Section vs Article as an example), but 99% the same as HTML 4 coding. There are a few new features, but those only come into play if you are taking advantage of them (offline storage, location, etc.).