They have a very powerful sting. So, if you ever plan on having corals (and other stuff you don't want stung) they're not so great. Because, as mentioned, they multiply like weeds, and for your corals it'll be like living in a mine field.
125 gallon saltwater: 55gal sump, 4 Blue-Green Chromis, Purple Tang, True Perc, Firefish, Royal Gramma, 5 Ricordea, Bubble Coral, 15 Pulsing Xenia, Green Star Polyps, Deresa clam, Green-Tip Torch Coral (about 11 "heads"!), Orange Montipora Cap, Purple M. Digitata, Green Slimer Acro. Yongei, 3 Orange M. Digitata, Pink&Green Acro. Millepora