quick search on google... im not sure myself becuase i dont use mac
How does Adobe Lightroom compare with Adobe Photoshop CS2
Photoshop is the industry standard for photographers, graphic artists and Web designers. But Lightroom is aimed squarely at photographers. Lightroom is also different from Photoshop in terms of its software architecture. Photoshop complements Lightroom when there√Ę‚�¨‚�Ęs a heavy image-editing task to handle. And Lightroom makes it easy to open a picture in Photoshop.
How does Adobe Lightroom compare with Apple Aperture
Adobe claims that Lightroom will support the RAW formats of more cameras than Aperture. Lightroom provides the option of storing your images outside the application in the file system. Lightroom has a Compare mode that lets you Command-click on thumbnails in the filmstrip to add them to the viewing panel for comparison.
Another important difference for Mac users is that Lightroom's system requirements are less strenuous than Aperture's. Adobe requires minimally a 1-GHz G4 processor with 768MB of RAM and only 1,024-by-768 screen resolution. The Windows requirements should be roughly analogous.
its still a pretty beastly program... i suppose i would still recommend picasa for the regular person who just wants something really simple to use