I kept one several years ago, and had I known then what I know now...I would not have done it.
They are really cool animals. Just as smart as an octopus, if not even more. But having one is like adopting a child. You have to really take care of their needs. It took me forevere to get mine to eat dead shrimp. Mine wanted live crabs, live crabs and most of all...crabs that were ALIVE. Talk about cost!!
Honstely, unless you have a big tank, 300+, I would not suggest having them. I know they look call, and they ARE cool. But geez...what a tough creature to keep alive. Mine lasted about 9 months.