I must admit, I love this extension. Being free makes it doubly awesome. What it does is so simple that there just isn't much to say about it in that regard except that it just works.
The one thing that would make it better would be if it borrowed a few features from the Chrome extension of the same name (which is a completely unrelated project). Specifically, I'd like to see it adopt two of the Hotkey features the Chrome extension provides: a hotkey to temporarily disable the zoom, and a hotkey to save the full-size image to your downloads folder.
The value of a temporary zoom cancel is that sometimes I don't want the images to be continually zooming when the mouse is hovering over them. For example, if I'm in a Google Image Search or browsing someone's Flickr Photostream, I don't want the zoom to keep trying to pop up every I pause while scrolling the page—I want to look over the thumbnails first to choose an image to zoom. Two types of hotkeys would help here: a toggle hotkey to turn on or off all zooming, and a disable-while-pressed hotkey to prevent zooming while it's pressed.
As for a save hotkey, that would allow one to bypass the need to load the next page to save a copy of an image locally. If I'm browsing, say, Deviant Art, with the current Safari extension, I can zoom the thumbs to get a good look at the image, but if I want to save it locally, I have to click through to the image's page to get the full-size image—saving by context menu will just capture the thumbnail. In the Chrome extension, I can press the save hotkey while hovering and it will save the full-size image locally without having to click through.