Summary: I heard once a wedding photographer had one of these but never found any use for it. This ended up being one of my key wedding lenses. My wife does the details and up close shots while I hang back getting the big picture images so I am a little partial to the wide images.
Conclusion: I carry lens with me wherever I go because it is so much fun to shoot and never fails to produces the most remarkable images. I've not owned any other fisheye lenses so I can't compare it quality wise with anything. All I know is the images this lens produces consistently puts a smile on my face.
Conclusion: The Canon 15mm f/2.8 Fisheye lens is a bit of a boutique lens, but can come in very handy in certain circumstances if you own a camera with a full-frame sensor. If your camera(s) has/have a APS-C or APS-H sized sensor, this probably isn't the lens for you.
Pros: Very sharp, rectilinear focal length, small size, does the job very well on cameras with a full frame.
Cons: No USM, nosier focus, lens cap, no full-time manual focusing, colors a tad muted than other Canon lenses I own.
Conclusion: I recommend this little toy. I bouth it after read reviews in this page, so I want to say "thanks" to who wrotte his words here. Check some pics with this lens in wedding photograhpy: http://www.facebook.com/photo.php?
Pros: Ultra wide. Fast. Sharp. Funny style.
Cons: No filter (painy scratchs danger everytime). Noisy like a rat.
Conclusion: What can I say... this lens rocks. I rented it for a weekend music festival where colour was abundant and this lens captured every little streak of it. In texture shots, it still performed very well and rendered pleasing shots. The colour saturation was mind blowing for me.
Pros: fun! wide! fast AF, crystal sharp, amazing colour
Cons: fogs over easily, fixed petal, lens cap, no filters
Conclusion: Contemplated this and the Sigma 15mm f2.8 EX Diagonal Fisheye. I even posted this dilemma on FM. From the answers I got, the Sigma won so I ordered that one. But at my shop, the Sigma was out of stock. They offered the Canon at a bit of a reduced price so I bought it. I am very happy with it.
Pros: Fun lens, sharp images
Cons: Cap, great care needed for front element, no pouch
Summary: Be it a circular or full frame version, the results of a fisheye lens has always intrigued. Several weeks back, I purchased a refurbished factory demonstrator from e**y with a UW date code and a one year warranty from the seller. The lens was pristine and generally sharper than expected.