Over the weekend a new build of Windows 10 leaked to the web. Build 9901 brings some pretty significant changes, and our own Paul Thurrott took the weekend to install it in a VM to get a good hands-on look.
After reading about Paul's experience with the new build, you might be enticed to try it out yourself. Paul made the right move by installing it to a VM because according to the Windows Insider chief, Gabriel Aul, there's a bug in the flighting (updating) mechanism that will keep you from being able to update to new builds when available.
@ZacB_ Yes, but not punitively. There is a bug in 9901 that breaks flighting, you'll need to upgrade through ISO.— Gabriel Aul (@GabeAul) December 14, 2014