I guess this title is a bit funny since this blog is dedicated to astronomy, so why I've to emphasis that? Please let me leave this question a little bit later, and let's have a look at my friend's result below:-
As you can see, the result is just not bad indeed! Notice that it's not taken with a real fisheye lens, but instead, only a cheap add-on lens is used. This kind of lenses are more widely used for consumer grade digital camera rather than DSLR, for a real fish eye lens will produce far better result.
However, as you can see from the above, the result could be good enough. Okay, let's get back to the first question about the title of this entry.
If you plan to buy a fisheye lens for special effect, I guess you should go for a dedicated/real fisheye lens. However, for astronomy purpose, you don't have. Why is that? To use a fisheye lens on the night sky, you don't really have a lot of flexibility on the composition, I mean you can point your lens to one or two different location and then you already cover all the sky! Therefore, a fisheye lens will not stay in your camera for too long given a new observation location, and after one or two (how about three?) shots, you will want to switch to another longer focus lens. Even if you take a few more for stacking, it won't stay attached to your camera for more than around half an hour. To me, it does not worth to buy a far more expensive dedicated lens. Of course, if you got the money and space, you can always go for the best.