Love it! It doesn't require a connection/account with Google at all. It's very useful because you have complete control over your pictures and organization. Just have Picasa plug in the folders you have your pictures organized in, and it will view all your pictures. You can add keywords or edit pictures, and they'll be saved/changed in their original folders. Very easy to use! Picasa isn't like iPhoto in the respect that iPhoto requires internal organization/storing of your photos, and doesn't save externally unless you manually export. Picasa is different in that respect, and gives you full control.
After setting your library up, just be sure to go to View and check "Small Pictures" and "Hidden Pictures" so 100% of your pictures are viewed.
I'm going to give OS X Photos a try when it's released. I won't be surprised if it takes the same structure as iPhoto/Aperture, but if it's truly a good quality, I might give it a go. We'll see!