My only grumble, and it's not a major one, is that Google are pushing the "premium" features of Picnik quite hard. So as you edit your photo, you'll find that a number of features are labelled as "Premium", and you might even get the occasional pop-up asking if you want to upgrade. Google is usually more subtle than that, and it would be nice if the premium features weren't quite so "in your face". In addition, it would be nice if, in time, some of the editing features of Picnik made it into the Picasa app itself, and weren't just available for online editing. Whether Google has plans to go in this direction only they can know, of course.
As an example of what you can do with Picnik, I spent about 5 minutes turning this photo from my Drop Box:
OK, not much of a picture I know, but a definite improvement on the original, and a fun five minutes.