Google Knol, supposed to be a Wikipedia wanna-be, has just launched. You can access it at knol.google.com
First impression of Knol is that it's far from being a Wiki clone. The interface's straight-forward - a few featured knols, search for knols or create your own.
The homepage looks like one of a Health Encyclopaedia. So far 90% of articles are health-related. There are definitely some great reads on them with some beautiful pictures & illustrations...(Yeah, do check out those articles by Tom Lue)
Most knols are more in-depth that on Wiki, mainly because the articles are written by professionals. Knol is more author-centric. Once a topic has been covered by an author, it becomes the author's property. Unlike Wiki, you can't change it, but only add suggestions which will be moderated by the author. Or else you write your own article about that topic. (Buttermilk Pancakes Recipe!! LOL) 
Anyone with a Google Account can create their own knols. The text editor's much better than that of Wiki, but not much external content can be embedded on it.
To provide much greater credibility to their knols, author can verify their names via phone or credit card. This is optional. Another plus-point of Knol is that authors can display AdSense ads next to their articles.
In short, Google Knol is much more a closed article platform instead of an anyone-can-edit Wiki - which is the biggest advantage & disadvantage of Wikipedia. But one catch with Knol is that let's say you write an article on “Orange,” you become the only source of fake information until someone else writes a related article or you are flagged! (Actually I was thinking of doing that, but I don't want to be the first spammer on Knol!) & the fact that it is integrated with AdSense means money-makers & spammers will be quick to flood Knol... Do check the Knol Help section for more info.