I had an Odontodactylus mantis for almost two years, unfortunately I lost her in a move. I will always remember her as one of my most favorite pets. She certainly was the smartest.
She grew to about 18-20cm long. She was always very curious and once she got to know me she would always come out to greet me when I came into the room. She ate just about anything, (sometimes she even got damsel fish for a treat! -- she just loved to chase them and they're more healthful than goldfish). But since she was a "clubber" she also liked to bash at mussels, turbos, etc. Another entertaining thing was to watch her clean house, she was constantly moving stuff in and out of her burrow and even bashing new tunnels in the rocks.
I highly recommend mantis shrimps as pets, but if you have a large "clubber", you might want to get an acrylic tank, a professor of mine has witnessed large Odontodactylus fracture glass.