Been running Picasa/Mac for a while now. It scanned my whole library quickly. I got it to crash by randomly clicking on things while it was importing, but it's been generally quite stable.

It recognized the Picasa.ini files laying around from before I switched to a Mac and did the right thing, although it apparently writes out new metadata as ".picasa.ini" (note the leading period) which means it's slightly annoying to go back and forth from a PC to a Mac using the same filesystem.

About the only annoyance is that it doesn't know exactly what to do with the "Originals" subdirectory made by Picasa/Win to save originals when you make some changes like red-eye reduction. Picasa/Mac just shows you the Originals directory. It doesn't figure out that it shouldn't show that.