bill - 4:13 am on Jun 7, 2013 (gmt 0)
This is aimed more at developers and people who want to be on the cutting edge rather than your average customer. The regular 8.1 update will essentially run just like any other Service Pack.
Apparently if you do opt for the Preview version then you're going to have to reinstall everything when the full version comes around, so the warning here is to be prepared to start over from scratch at that time. Most people following this path would likely do it on a virtual machine or a system that is not primary to their work. I certainly wouldn't want to take the time to reinstall all my software and system settings on my main machine.