I had a lot of trouble exploring picasa and picnik. I wanted to put some pictures up on the web (see the Cat in Lucca, the painting from the hotel, and the Leaning Tower of Pisa) but picnik took a very long time to upload the pictures from my laptop to the website. I had downloaded all of my pictures (23 of them) with Picasa before picnik had finished downloading one picture. I would recommend using Picasa to our customers over picnik (if I'm ever asked) because of this time difference.
However Google has some interesting Terms of Service that I'm not sure are appropriate; Google explains that the Terms of Service were written in English and if there's any difference between a foreign language translation and the English version the English version is correct and a user will be held to the English Terms of Service. I don't feel comfortable pushing a service that is that unfriendly to people who don't speak English fluently enough to read (and understand) Terms of Service in English.