Far more useful than you might think, really fun to shoot with and the images are razor sharp. Check fotosidan.se for photos taken with the lens, but for catching tight situations, like indoors or spectacular landscapes, this is really good. And fun.
The only negative is that förvrägningen in image edges in many cases become too excessive, of course depending on the subject and composition. I'm not an optics freak, so can not say whether it is inevitable with fisheye concept, but have experienced the annoying in some situations.