One drawback for me with iPhoto is cropping. I would create an album for getting 4x6 prints, crop all the photos, export and send to photo lab. However, all those photos would be cropped in the main library and unless you use revert to original, and lose your other edits, you can't undo. There have been several cases where I want to use the same photo for multiple projects at the same time and require different cropping levels.
My original solution was to make all the edits to a photo and then use iPhoto Diet to replace the original with the modified photo. Now after cropping, a revert to original retains the edits but removes the crop.
My new solution is using the sharing feature with fast user switching (FUS). I create an album in my account for that particular project, and add the photos. I enable sharing for that album. I then FUS to a user account set up just for photo projects. I can then create an album and drop the photos from the shared album into a new album, which creates a copy of the photo. I can now crop to match the project without affecting the main photo library. I can then delete the album and photos from the project library when finished to free up the space.
If I want to keep photo projects long term, I could use iPhoto Buddy to create a separate library for each project on the secondary user. Then just switch between them depending on what I need to work on at the moment. I could also archive the library for each project separately.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040702101846434