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*
They really are very cool, but a real pest in a tank. They grow to coat everything in the tank, even to the point of making it hard to see the fish. Some fish eat them.
They're actually not all that easy to find anymore, what with agricultural runoff and such. NATURAL ponds with lots of algae in them tend to be good places to look.