It'll take at least a year (maybe two) to ship EPUB3.0-ready devices. A large-ish, ongoing concern is with scripting. How would the security model for that look like?
Granted, Apple's recently gotten involved in the EPUB3 working group, and that may force some pressure on the group to ship. But the spec is rather vague in some areas, and I expect it to take some time to hash out.
I'd say that KF8 is equivalent to EPUB2. Maybe a wee bit better. But my fear is that it's good enough to warrant switching over from EPUB.
Does KF8 support JavaScript? I don't see why allowing scripting in an e-reader app is any different from allowing it in a web browser app.
I also don't understand why you think it'll take so long to ship an EPUB3.0-ready device. The current iBooks app already allows for some embedded JavaScript (which only came with EPUB 3.0). Kobo says they'll have an e-reader device that supports EPUP 3.0 in the next 3-6 months: http://www.digitalbookworld.com/2011/five-tablet-trends-sign...
Well, you ain't seen nothin' yet!
On October 11, the IDPF membership unanimously voted to elevate EPUB 3.0 to a final IDPF Recommended Specification. And what's in EPUB 3.0?
XHTML 5, SVG 1.1, CSS 2.1 and 3, JavaScript, TrueType fonts, WOFF fonts, SSML/PLS/CSS3 Speech, SMIL 3, RDF vocabularies, MathML, and more...
Compared to EPUB 3.0 (parts of which are already supported by Apple's iBooks app), Kindle Format 8 is behind the times.