effective for what?
They are just a tool to use in the weight room. I don't think that they are any better than anything else. I don't think that using them will lead you to athletic success and that they are the answer to everything.
They are a tool. They are nice to have. Pretty much everything I can do with a KB I can do with a DB (although it would feel a little difference).
No matter what, DB or KB, the same rules apply:
progressive overload
work hard