Can only agree to other commentors: great start for a free utility. Easy scan, and especially in times of PhotoStream, there's always a good chance of duplicates.
That said, I't mid term really like to see the possibility to find (and subsequently remove) duplicates of modified files. E.g. when rotating an image, or performing some image enhancements in iPhoto, iPhoto will keep the unmodified original (in order to always be able to revert back to it). It would be an incredible space saver if one would be able to delete the originals.
But I realize that this might require a scan on file system level, whereas iPhotoDuplicateCleaner performs its task by using AppleScript from all I can observe.
But anyway, already great the way it currently is. Thanks to the developer for making this available for free!