In case you haven't already noticed, the Feb CTP of Windows Vista has been posted to MSDN Subscriber Downloads.
I've been running various CTP and internal daily builds for a while and this is by far the most stable and if the last week is anything to go by very stable and completely usable - in fact I've been writing chapters of the book today and not lost anything :)
I love the new SideBar, it doesn't take up loads of real estate, the new Aero look and feel looks great and the picture viewing/gallery stuff rocks (at last you might say!). XP looks so dated now :)
Things to watch out for, Office 12 Beta 1 isn't very happy so use Office 2003 instead, Messenger v8 Beta doesn't install right now, and if you've got a tablet rotation might not work (depends on your hardware).
This release is now feature complete so pretty much all the features are visible in this build.
If you want the compatible WinFX runtime and SDK to start developing/playing with WPF,WCF and WF then you want the newly released Feb CTP builds that are here