Mantis shrimps sometimes referred to by some as the 'roach of the sea' are definitely hated by some but valued by others.
I have three specimens in my tank and they are quite interesting.
Though the large variety which can grow to 5-8 inches long are reputed to be able to shatter glass with the blows from their hard calcified forearms, my two larger critters (about 2 inches long each) are harmless in my tank.
They are extremely cautious and rarely venture out of their burrows. Instead they sit at the entrance with their two eyes,
which many describe as displaying uncanny intelligence, peeking out.
If you have small snails in your tank, they may become food for the mantis shrimp, as mine can often be heard pounding away at some snail until it smashes its shell with its forearms.
And yes I too think they are cool little creatures!