It may be improved, but Quality Assurance (QA) only kicks in Release Candidates. Up until then, the builds are buggy and unstable. So I wouldn't recommend overwriting Vista with it.
It really looks like an enhanced version of Vista currently. I would love to use it, going to download it now..
The first few builds rarely focus on the GUI. For example, here's Windows Vista M1:
Edited by ShamusHand, 02 May 2008 - 03:22 AM.