And how does Picasa 'do this'? The issue with all of the photo managers out there is this: How to I go through my photos and tag the good ones as 'good', and tag the bad ones as 'bad' and if I quit and resume, how do I continue with the untagged (or continue from where I left off - considering that this may take weeks or months).
PhotoSorter isn't about eliminating duplicates or managing your photos - its about reducing an unsorted collection to a manageable and shareworthy collection.
Consider this - I've got over 25000 unique photos, never really been looked at or sorted. Nothing ever deleted.
I could use Picasa to go through them individually and tag/upload/share the good ones, but I'd have to remember where I was up to.
I could set up tags Good & Bad and tag them all like that and if Picasa could show me all the untagged photos that would help (help me continue from where I left off).
What PhotoSorter does is simply this - step through each photo one at a time and flag it as a keeper or not.
You can exit and resume from where you left off at any time, and export the flagged photos at any time.
So I'm using it as a convenient way to reduce my crazy photo collection to one that is worthy of loading in Picasa and manage/share there.
Right now I'm exporting what I've tagged every week (exported to yyyy-MM folders). Then, Picasa is configured to include that export directory, and those yyyy-MM directories are set in Picasa to sync to Google+.
For the first time ever, my family overseas gets to see my photo collection - without the crap. And, although I will still back up the entire collection (all 50k photos incl duplicates) my exported (good) photos will be much smaller and easier to back up (I have several backups, disk, bluray, online, offsite).
Also, because the collection in Picasa is only the good ones, I am quite happy to set it off in slideshow mode, and know I'll enjoy it.