They are very cool in a zoology class. You don't want them in your tank. I got some in one tank and it took DRASTIC measures to remove them. Mine never got big, there are many varieties and some get big enough to actually catch small fish. I guess the ends of the tentacles and anesthetic/paralyzing stingers. Then they divide and grow and grow some more and divide and grow.
Keeping one in a mason jar or something could be fun and interesting but it is not something I would every consider intentionally putting in a tank. You just can't ever get them back out. *shuddering*