I have used iPhoto for several years, and thought I'd give the competition a try. I've listed below some of iPhoto's shortcomings, which resulted in my switching to Picassa. I'm tired of submitting feedback to Apple, and with each new release of iPhoto (and iMovie), critical functionality is still omitted.
1. iPhoto photo management is horrible. Other photo management software such
as Picassa or Lightroom allow you to simply import a top level directory,
after which subdirectories with their meaningful names are displayed in
the application. No such directory hierarchy awareness exists in iPhoto.
2. Photo management with Picassa is automatic. If I add or delete individual photos,
or subdirectories in "watched folders", they are automatically updated
3. Picassa and Lightroom are referenced libraries, and work extremely well
in that capacity. iPhoto does not work well in that capacity, and I've
read many recommendations in these forums to NOT use iPhoto as a reference
library for that reason. Instead they recommend that you import all of your
photos directly into iPhoto. The problem with this is that iPhoto will
eventually become bloated and painfully slow.
4. Picassa allows you to take still snapshots from videos. Brilliant! iPhoto
never had this ability. iMovie actually did have this feature, but Apple
in their infinite wisdom decided to remove the ability to take still
snapshots from movies. The "workarounds" recommended in numerous forums
is to take a screen shot, or use MPEG Streamclip. I still cannot believe
Apple removed this critical functionality.
5. Picassa allows you to view your photos metadata.
6. Picassa allows you to select a start point and end point in a video and
export just that small clip. Nice!
7. Picassa is free and runs on Mac, Windows, and Linux.