I really liked Sundiver, Startide Rising and managed to enjoy Uplift War. But the concluding trilogy just got to be too much for me.
Re Clarke's Childhood's End, the big reveal about Karellen was a silly touch that pulled me out. It was way too much like those SF stories where two of the characters turn out to be Adam and Eve. Pretty much everything else by Clarke I liked better, even A Fall of Moondust or The Sands of Mars. (But not Earthlight.)
Scalzi did move away from the Heinleinism so far as ideals go, which greatly improved the stories. If you want a Heinlein pastiche, I think Michael Kurland's Princes of Earth is probably one of the best on offer. I also think that the rationale for old recruits fails in the face of the practical rationale for young recruits: They are naive (especially if they're convinced they're street smart,) and much more susceptible to indoctrination. I liked the Scalzi novel about the galactic sheep better than any of the old man stories.