I always felt that he really got Superman. He understands the character, what makes him Superman and the physics. Kingdom Come really put emphasis on how 10 years of absorbing Sunlight had made Superman stronger. This is only simple physics for Superman but no previous writer really put emphasis on it. For example, Superman was not very powerful in The Dark Knight Returns, despite the fact that he was 55 years old and should have absorbed alot of sunlight.
Furthermore, Waid has written two Superman masterpieces: Superman: Birthright and Kingdom Come. One story dealing with Superman's origin and another his return (similar to Frank Miller with Year One and DKR for Batman).
But Waid is really a great writer, I wish he could write more in the New-52 (has he even wrote in the New-52?). Writers like Jeph Loeb do not understand Superman (he wrote on good story which was For All Seasons). Loeb always seems to make a Superman story stupid, and don't get me started on what happens when he gets hold of Supergirl.