I'm running with a Mk1 Lightning and am pretty happy with it so far. There is a twang there but it isnt that bad, the later rifles might be a little more twangy but others will be in a better position to answer that as I havent owned one. I have used it on rabbit, squirrel and rat since I have owned mine, which hasnt been long and have had no problems dispatching any of them humanely.
I like them but it is all subjective, what suits one person may not suit another. Try one if you can