basically there are two distinct ways polys behave when gone dead -
1. you start spraying balls, they become sort of a trampoline - kirschbaum pro line II, most isospeed strings
2. the ball literally sticks to the strings and you have to start overhitting in order to get that darn ball over the net - luxilon alu power, solinco revolution.
the latter are the most dangerous for your joints, the others only might get you to pay for some fence damages
the most decieving ones also are the latter mentioned, since you initially think you have more control in the sense that no matter how hard you hit you still make the court. the best indication is when you have trouble raising your hand to drink the breakfast coffee/tee the next day.