Really, I'd advise against it. It isn't ready.
The reason Apple have these developer preview releases is so that developers can write apps that take advantage of the new APIs. That means that when it's released to the public, they'll have apps that integrate with the look of the system and take advantage of the new features from day one. It's also important for finding bugs (and believe me, there can be big ones!)
That said, the preview releases (especially the earlier ones) are typically really buggy. Don't be surprised if it crashes every time you try to make a call, WiFi doesn't work, or any other catastrophic things like that. If it was ready it would be shipping today.
At the very least, you should wait to hear what the stability of the preview is like before installing.
This is mostly balderdash. I've had a developer account since the beginning, and loaded the betas immediately upon release. There are bugs here and there, but I've never had an issue with any of the core functionalities (phone, text, email) or core apps.
It's a beta release, not an alpha or pre-alpha build. It's pretty much > 75% of the way of being a final release, and most people can deal with a little bugginess as a trade off for the vastly improved (and long overdue) OS overhaul.