Crabs: To have or have not

[Sea Witch]

I've read all kinds of opinions from one end of the spectrum to the other:

"Crabs are terrorists--don't let any of them in your tank." to
"I love my crabs, they don't bother anything, and they help keep everything clean."

So tell me, do you have any crabs in your tank, what kind, how many, and do you like them or not? Do they do damage or not? And how big is your tank?

Thanks all,