Picnik is still a Flash application and it doesn't load quickly, so you have to wait at least 10 seconds until you can use it. The online photo editor doesn't have all the features that are available in Picasa, but there's an useful "auto-fix" option similar to the "I'm feeling lucky" button from Picasa, you can crop photos, repair red-eye flaws, remove color casts automatically and fix exposure.
The nice thing is that you no longer have to sign in to Picnik and that the changes can be saved to Picasa Web Albums after you click "save to my album", but that's nothing new. Flickr has been offering a similar feature since 2007.
"Picnik's rich editing tools are now integrated into Picasa Web Albums allowing you to experience them without ever having to leave your account. As long as you're using Picasa in one of the Picnik supported languages, just click 'edit' from the edit drop down menu or from the new handy Picnik icon. Then, Picnik away by applying an effect, adding a sticker, or exploring your own creative path with advanced tools. When you are done editing your photo, save back to your album by either replacing the existing image or making a new copy," suggests Google.
I expect that Picnik will be rewritten as an HTML5 application, so that it can have a better performance and properly integrate with Picasa Web Albums. Instead of opening the editor in separate box, Picasa Web Albums should let you edit the photo in place.
{ Thanks, Sterling and Niranjan. }