This is a side-by-side image of Windows Vista 5308 sidebar next to a 3rd-party application known as "Desktop Sidebar". Both are running on my desktop simultaneously and are docked to the right on my 16x10 ratio monitor (a great reason to spend the bucks on a widescreen monitor).
I use Desktop Sidebar as my main application for shooting thru 90 or so RSS feeds. I've been using it for >year and it's very stable (a beta version is also available that is not fully stable). It is freeware... and all that implies. But I'd call it freeware-lite since it comes from an established company of developers, versus an single individual. It's got a development methodology behind it.
Windows Sidebar just surfaced for the first "official" time in Vista build 5308. I really wanted this application to work well because it's visually stunning. It's also all about looks and currently offers very very little worthwhile function. I'll give MS (and a former acquantance there on the dev team) the benefit of the doubt for now because I know what it took to get it this far. And if it continues on this track it wil be a winner.
But there is so much more needed... I can't use it for "prime time" at this point in time.

Clocks are very similar.
Slideshow of pictures is very similar... except in Windows Sidebar there is no way to change how long the pictures are shown.
Performance monitor in Desktop Sidebar is very useful... you can even plug in things like disk temps (which, with 4 drives spinning in my uber-box, is very useful).
The "launcher" in Windows Sidebar isn't very useful to me. I run a very large number of applications... it'd never be so simplistic for me as to just launch the same old 4 or so applications all the time.
But here's the real rub - the RSS feeds. There is no comparison, IMHO.
Desktop Sidebar can apparently handle any number of feeds - I have about 90.
Windows Sidebar can apparently handle any number too - I put about 10 into it to start. But, how to switch amoungst those feeds is intuitively impossible to determine. How to configure those feeds is intuitively impossible to figure out.
And, worst of all - the size of the feeds display is pretty much fixed... whereas Desktop Sidebar can be modified as needed and the ultimate limit is only the 10 or 12-font size divided into the height of the desktop window. And even then it will scroll. And it has a skin editor so I can do pretty much whatever I want to it's appearance. It does not take any advantage of Vista whatsoever - it is not even aware of Vista.
In terms of plug-ins, Desktop Sidebar has dozens or even hundreds...t here has been a very active community all along.
Microsoft realized the need for this and has a funky and strange website where people can development and put them up for download. Only problem is that most of the ones up there so far are pretty much pointless or silly. Many of Desktop Sidebar's plug-ins are too... but it does show an incredible enthusiasm and variety.
One mor epoint - Desktop Sidebar is well-synched to my latest posts. Note int he example image that Windows Sidebar is showing a bunch of old posts... how it gets updated is not intuitive. Note that IE7 and Outlook 2007 on the same machine are bothy showing the most current feeds correctly.
So, I'm stuck... an application I really wanted to promote is not useful to me at this point. MS has another 6 months (give or take) to go before Vista is realised. I hope somebody takes a crack at an industrial strength RSS reader before then.