Behdad and I prefer to have the "Original" size of a photo to view, or use (if it's under a CC lisence). So we wrote a Bookmarklet to make it easy to save the original size of an image on Flickr.
Here is the Flickr EasyDownloadr bookmarklet.
HOWTO: Right click on previous link. Bookmark it (preferably in Bookmarks Toolbar Folder and rename it if needed). Open the Flickr page of the image and use this bookmark to save the original size of the image.
UPDATE: And here are the Flickr EasyOpenr which just opens the original size.