It depends on the lens. Some are quite good and some are not worth the effort.
In general, a fisheye lens has limited usefulness. Because of the extreme angle of view (180+) the distortion is inevitable. My primary use for add-on fisheye lenses is to facilitate quick 360 degree panos (QTVR) for clients in real estate. Software is available to "defish" the images and do a tolerable job of quickly making a 360 degree QTVR with only two exposures. To my knowledge, only Raynox makes a suitable fisheye which can be adapted to the F828. Nikon makes a couple models for their digicams, but one is not suitable for larger lens digicams (made for the 28mm filter thread CP series) while the other "may" work. You would need to check the specs at B&H or somewhere to be sure.....
On the other hand there are plenty of add-on filters of excellent optical quality which can be used to great advantabe for macro work with a fixed lens digicam. With my F828 I use a Nikon 5T closeup filter which works very well. Some very serious macro shooters use the Canon 100mm F2.8 macro lens with their digicam by making their own adapters. It works very well with the Panasonic FZ10 and FZ20 to allow some incredible closeups while maintaining a good working distance. In other cases reversing a 35mm platform lens in front of you fixed lens can result in some excellent extreme macros.
If you want more information on the macro end, here's a link where some very talented macro shooters use these combinations. You can pick up some great information here:http://photomacrography.net/