I sometimes enjoy the creative challenge of being constrained to a single lens, and it is certainly a challenge to create portraits with a fisheye lens. Fortunately, my kids get the humor of the thing, and play along.
Besides the obvious distortion and curvature, the key thing to bear in mind with a fisheye is how much close foreground it includes. In the landscape context, this implies that you better have something interesting in the foreground of a fisheye composition (consider my Between the Earth and Sky as an example).
Moral: if you are taking portraits, you need to get the lens really, really close, like an inch away.
For more fisheyes of my tolerant kids see Cruel & Unusual Lens.