Check out the full review of Sony's 16mm F/3.5 fisheye converter. This can be a fun lens, or an awful lens, it all depends on your level of creativity. My level is pretty low at the moment, confirm this by going to the middle of the review and looking at dumb pictures, like inside a washing machine, oven, or the fridge.
The fisheye is actually pretty good, and certainly worth the money if you have 28/2 already. Buying both is pricey, and a long and heavy combo, read the review to see if it's right for you.