I have been using the Windows 7 beta as primary os on my main machine ever since it got released. Over all I like what i see and I plan to move to it when it gets released.
Some of the new features that i like are:
Stability and Speed… The beta overall feels quite stable and fast compared to the release version of Vista. But since its a beta tho there are bound to be bugs and i have found afew of them such as explore crashes that require reboot for it to be fixed.
New tasbar… The new tasbar helps with my productivity since im no longer gettting packed taskbar like I would get on XP/Vista. It takes getting use to as per the non running apps pinned to it. The effects are not that bad… not too flashy but functional
New Aero features… Aero peek is quite handy for a multi-tasker like i am since i can see the actual app where it is on the screen instead of a tiny thumbnail. Aero shake is so so not as flashy as peek yet is quite usefull. Now Aero Snap rules when i go to xp/vista i really miss that feature.
New System Tray… I like how it defaults to hiding the tray icons by default vs how xp/vista does it. I like a non cluttered taskbar 🙂
Now i better stop before i start rambling on more and more :).