I use spitfires and I was really impressed with them, the doe I killed last year ducked/jumped the string and it hit the tip of her shoulder blade, it cut right through it and deflected up into the spine and shattered it! She dropped like a rock. As 5 shot said KE is important, and as others have said it is just as crucial to have a well tuned bow with mechs as it is with fixed heads. To me the biggest advantage to mechs over fixed heads is they will NEVER plane on you and the wind has far less of an effect on them. They are also quieter then fixed heads in flight.