Are Kettlebells Really That Great?

Can kettlebells maintain (and preferably, increase), brute strength along with flexibility, agility, and endurance. If kettlebells are really that good I will find the money (I'm a student so that is saying a lot), but I need an honest opinion as to how effective they are.

Through their use can I turn myself into that balls to wall non-stop terror for a full five minute grappling round, recover, and do it over and over for as many rounds as it takes? Also, how many kettlebells do I actually need?

I added up how much it would cost to buy two of each weight and that's a lot of money to me.

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