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.
Oh, but were it not for Lendl, Murray would have never improved his serve. Take Del Potro for instance. Everyone in Argentina is saying the areas he needs to work on in order to critically enhance his game, but neither he nor his coach seem to listen. The same occurred to Murray before changing his coach.
Lendl is the reason why Murray is so successful now.