I tried it only with the official emulator, not with a real device.
Ah, maybe thats because we had different experiences.
Looks like Google started with the internal things and now they have to focus on the UI and fix upcoming bugs of the internal system.
Another thing is that not all things they showed at the keynote and in the I/O sessions made it in the preview release. There is much stuff just missing. So it is really a developer release. Everything you need as a developer (ART, new APIs etc.) is in, end user stuff (like user detection by other bluetooth devices) is not.