He's a good coach.
But lets not get carried away.
Every tom dick and harry in Scotland was saying that Murray had to improve his serve, especially 2nd serve, and be more aggressive in the points and rallies before he even got appointed as coach.
Difference being hearing it from Lendl made him listen as he is a legend of the sport. Petchey (LOL), Gilbert (junkballing mega-pusher) and MacLagan (yes man) were never gonna have the know how or accumulated respect to guide him like Ivan can.