I tried Picasa, iPhoto, Bridges (adobe) and Aperture. My (noob) impressions:
Aperture: ...Lets say you are browsing some photos and you want to copy a photo to desktop; Instead of dragging the pic (just to copy a small res copy), you have to click export/versions and choose where...
You can turn on automatic generation of previews upon import, or by project, simply select the project, then click the gear icon and check the "Maintain Previews for Project" item.