I like the Olympus 9 - 18 mm zoom range better than the 8 mm fisheye, but I still like to using the fisheye lens. The big problem with all fisheye lenses is that you need to get very close to fill the frame and it can be hard to compose in the smaller view-finders.
I like to use the 8 mm fisheye with the X-1.4 tele-converter and extension ring. This makes the lens an 11 mm fisheye and allows a little more distance between the dome port and the subject, as with this image shot at ISO 100, F/5.6 & 1/90th sec. from about two inches to the subject. Without the tele-converter the dome would have been in the eels mouth. The 8 fisheye has three dome choices with the Inon dome being the least expensive, however this dome only allows use of the fisheye lens (other zooms will not do well with this dome). The Olympus/Athena/SeaTool 170 mm and Athena 220 mm domes also work very well with this lens as well as the 7 to 14, 9 to 18, 11 to 22 and 12 to 60 zooms.
If you make the Olympus 8 mm fisheye your only W/A lens combined with the Inon fisheye dome the total cost will be about equal to or a little less than the cost of the 9 to 18 mm lens, 170 mm dome and zoom gear.
Phil Rudin