iPhoto hasn't had a great free competitor and in my opinion, Google's photo management is just, well, better. That's why Picasa for Mac is awesome news.
Feature for feature, Picasa for Mac is almost exactly the same as its Windows and Linux counterparts. The organisation paradigm is exactly the same, as is the interface (which, it bears mentioning, somehow still looks natural in OS X). Even the automatic system-wide photo indexing worked fine, as did a few different camera imports.
The program still carries Google's increasingly meaningless "Beta" tag, but judging by my brief testing, it's ready to go. [Google—Thanks, Ryan]