I personally do not feel comfortable with requiring a script to tell a browser how to read plain old markup. Goes against the whole "device/software/platform independence" HTML (and the web) was originally designed for.
I've been reading about learning HTML 5 & CSS3. Currently, it seems great that it can create all these wonderful effects in every other browser expect IE. At this point in time, it seems a nightmare to work with something that isn't completely ready and fully functional.
Well, what's "nice" about CSS3 is, so long as you don't do anything stupid like relying on text-shadow to show white text on a white background (that kind of thing), it's just an extra: browsers who support the particular CSS3 property show it. Those who can't, don't. Your basic, non-CSS3 site should work in all browsers (so test it), then layer whatever CSS3 fancy junk on top you want. It's usually not a deal-breaker (what it really depends on is, does you client insist certain effects are cross-browser? If so, no CSS3 for you).
I don't know about yourselves or your opinions, but it feels like we're the guinea pigs for a lot of these technologies and it can be added unproductivity.
On the other hand, things in draft need real-world testing. This is why Perl6 was released as "Rakudo *"... Perl6 is not "finished" (though it's usable), but this is in part because the designers of the language need to know what do developers want? need? use? don't use? find more bugs in?
And of course you can just not use these technologies, especially when you need to be "safe" for clients.
It brings up another point, if we're just using HTML 5 to change a tag from <div id=header"> to <header> its not really a life or death change.
No but it does add unnecessary work today. Browsers who don't understand HTML5 need to be told <header> is a block. They don't need to be told <div> is a block.
The JAWS screen reader would ignore anything inside <header> tags if the user was using Firefox until the recently-released JAWS 12 came out (and since it costs an arm and a leg, unlike a browser, you betcha there are going to be users with JAWS 9, 10 and 11 visiting your site for a while yet!).
Window-Eyes also has some trouble with HTML5 elements, sometimes especially when adding ARIA attributes (and the funny thing is, the extra semantics that HTML5 is supposed to give us hasn't reached AT yet, so right now <header> doesn't mean anything more than <div>... that'll take some time yet... so right now HTML5 needs help to be accessible at the moment).