All the people who got introduced to the wonderful world of ambigramming through the ‘Angels and Demons’ you might want to check these links.
As Paxy says, a good ambigram must follow the criteria of : Symmetry, Legibility, Aesthetics.
That’s what makes it a piece of art, isnt it? Why wait then? Go ahed and do some ambigramming. Someday I shall do one good ambigram. What about you?