First of all, Aero Peek had to be enabled in the previous builds of Windows 7, whereas it's now enabled by default. Once you're using it, the main change in the latest build of Windows 7 is a new fade out feature, which highlights the window you're hovering over by fading out all other active windows on the desktop—so not only do you get a window preview in the "peek" window, but you also get a full-on preview of the window as it looks on your monitor. The video below should give you a better idea of what I'm talking about.
NOTE: Unfortunately the video ended up a little choppy, but the Aero Peek feature is actually very smooth in the Windows 7 Beta, complete with animated transitions between every aspect of your window peeking.
Now that you've had the chance to install Windows 7 on your PC, how do you feel about Aero Peek? Great new feature or tiresome eye candy?