I think he had some really good ideas in him, but wasn't always great at execution, and had some pretty lame ideas too. I think "The Cage" is a solid piece of science fiction and an effective character study of Pike. Yet many of his credited scripts or stories for TOS are loaded with silly parallel-Earth stuff ("Bread and Circuses," "The Omega Glory") or rampant sexism ("Mudd's Women," "Turnabout Intruder").
And he certainly had lost it by the time of TNG. The first season, which he heavily rewrote, is full of stilted and awkward dialogue. "Hide and Q," one of the few he got screenplay credit for, has got the most painfully awful dialogue in all of TNG.