I've only seen a clip of one Roddenberry-penned HG,WT episode, but I felt it was pretty smart and well-written. The TMP novelization? Interesting, but awkward. It's recognizably the work of a screenwriter who's never done prose before. It's imaginative in its futurism, but reflects Roddenberry's sexual preoccupations.
also has some interesting ideas, but in many ways it's an awkward pilot conceptually and would've been hard to sustain as a series. The future society PAX is a little too forbidding and hard to like, despite its noble ideals. The biggest problem is the portrayal of Ted Cassidy's character Isaiah as a "White Comanche," wearing brownface and talking and acting like a stereotypical TV-Indian savage.
is a better pilot in a lot of ways, but it was co-written by Juanita Bartlett, and she manages to temper a lot of GR's excesses, so that the plot of Dylan Hunt using his masculinity to win over the head of a female-dominated society turns out more humorous and mutually respectful than the masculine fantasy it could've been.
The Questor Tapes
is also rather good, but it was co-written by GR and Gene L. Coon, and Coon was a very good writer.