The founder of the Mountain Ocarina company, Karl Ahrens, recently put up a very nice video:
http://www.youtube.com/watch?v=XUkGAa6J-H8
I have C and G Mountain ocarinas, and I find that I prefer the C, but the video shows that in the right hands, the little G can make big music. For me, the appeal of these instruments is that they are small enough to fit in a pocket, and also indesctuctible, so I tend to take one with me everywhere. I prefer the whistle for Celtic music, but it’s interesting that Karl is very keen on playing Celtic music on the ocarina (in other video clips and sound samples). The ocarina doesn’t have the whistle’s range, by a considerable margin, but if you’re inventive about transposing things into the key of C and shifting parts down an octave when needed, you can play quite a bit. Personally, I tend to reach for the ocarina when I feel like playing things other than Celtic music, but that’s just me.
Anyway, enjoy the video.