I have chronicled, on my own blog, my wish for Picasa for Mac for about 3 years now. As you can see, the post continues to receive comments and remains, to this day, one of the most visited entries on my site. Clearly, there is demand for this product.
What I believe makes Picasa such a successful product is just how powerful it is. Although iPhoto works very well on the Mac and the iLife integration across applications is priceless, the fact remains that for serious editing and effects, the Mac user must venture outside of iPhoto. Picasa, on the other hand, has an entire suite of tools for photo finishing. Furthermore, Picasa features Google's search tool, a bevy of organization tools, a plugin system using "buttons," out-of-the-box integration with Gmail, Blogger, Picasa Web Albums, and the ability to make collages, movies, and more. In fact, there is little doubt that Picasa is a much more robust application that iPhoto.
There are some missing features in this beta: Geotagging didn't make the cut, nor did webcam capture, screen capture, and screensaver. Also missing are the ability to order prints, an HTML export, and the fantastic Picasa Photo Viewer. Most of these features are certainly tied tighter into the OS, and while they will be missed, they are by no means deal-breakers.
What remains to be seen is whether or not Picasa is stable, whether or not it's fast, and whether or not it can handle large photo libraries. I know people with well over 15,000 photos in their iPhoto collection, and the application is solid. Since Picasa doesn't store it's own library, but rather, merely catalogs photos elsewhere on your disk, we'll have to see whether this translates into a performance advantage or disadvantage. It remains to be seen if Picasa for Mac can go toe-to-toe with more mature, native solutions. That said, count me in as one of the many waiting to find out.