I have a Paperwhite. I transfered more than 1,000 mobis and they seem to show up just fine. Indexing does not seem to be happening, though--whenever I search for a line of gibberish like xcsgez, it shows the same number that are still not indexed. Normally I should see it progressing through the files steadily by now.
Plugins, unsurprisingly, don't work yet (Kindle Collections and page numbers). At least, I don't see any page numbers though, come to think of it, it DID tell me what page number I was on under GO TO. It just doesn't display that page number as far as I can tell. But I haven't played with the device much yet.
As Kovid says, I hope Amazon will minimize the silliness. They do the big things well, them get crazy on some of the smaller things, such as making you remove dmr to left-justify your text. Why not allow people to have that decision on Amazon books?