FCPX has dropped into the App store along with Motion and Compressor. Those links are App Store links, btw.
Many initial questions are covered here. It's well worth a read.
Macworld has an article on it here.
However, right off the bat with the dot zero release:
1. You cannot currently import FCP6/7 projects into FCPX (see #4)
2. There is no Multiclip/Multicam.
3. Many plug-ins currently do not work.
4. Currently there is no XML Import or Export.
5. Currently there is no native OMF Export or Export to Logic.
6. Doesn't currently work natively with R3D or XDCAM (they need to be re-wrapped first).
An update has already addressed this somewhat.
7. There are no longer distinct Canvas and Viewer windows.
8. You cannot set In and Out points for tape capture nor (AFAIK) is there a batch tape capture ability.
9. You cannot "Save As" to duplicate a project (at least not that easily).
10. Apparently you can't Print (output) to tape, anymore.