Very very early impressions: We didn't expect it to be, and make no mistake it is not ID.
Good News
Works just like Keynote and there is a lot to like.
It is well-organized and mostly intuitive.
Doesn't have problems publishing, doesn’t require deep knowledge of ePUB ins and outs, publishes free without DPS.
Bad News
Styling text is not great. After you've had InDesign you've had the best. Styles editing needs to improve—vastly. I mean, no baseline grid, shoot me now.
Styling text is inconsistent and lacks hi-end features (nested styles, glyphs for custom bullets, many more). I created a style. I put my cursor in a different graf and tried to invoke the new style. There were overrides, it didn't work, and I couldn't figure where to clear those overrides. BLECH!
Apple offers these great click & edit widgets, like slideshows, media, html, 3d … but when you drag them on your page it lets you know that if you want an html widget, it's not an html page you're loading from Finder — no, it's a Dashcode element created in some other Apple app. Same for 3d, it does not import the standard dxf or obj or 3ds file formats, it wants a Collada .dae file. (what the frack?!!)
Draw a text box and try to import text — no, it doesn't work that way. You have to Insert Chapter! WHAT!?!? So it's back to the caveman days of cut & paste.
Who is it ready for? It's prime-time for the person who wants to type into Apple's templates.