A couple things to keep in mind with MyPhotostream: The app can show only the most recent 1000 photos in your stream, and since it's merely accessing where the stream is stored on your Mac (in a folder in ~/Library/Application Support/iLifeAssetManagement that you shouldn't touch), it's not doing any additional archiving — for that you'll still need iPhoto or Aperture. In fact, you'll need iPhoto or Aperture either way, as MyPhotostream requires that you have one of those apps installed and that you've configured Photo Stream in that app or in System Preferences. After doing so, however, you can use MyPhotostream without having to touch Apple's own apps.
Also, MyPhotostream handles only your main, personal Photo Stream, not those shared with you, or by you, and it can't be used to upload pictures to your Photo Stream — only to view them. (That said, if you open one of the photos in, say, an image-editing app and tweak the picture without making a copy, those changes could cause problems with the photo-sync; I wouldn't recommend doing so.)
While having those capabilities would make MyPhotostream even more useful, more features of course means more complexity. The utility and joy of MyPhotostream as it stands is that it's simpler than the alternatives — and for that, it's a welcome addition to your Mac's Applications folder.
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