I finished "Mission of Gravity" a while ago, and have had some time to think about it since.
This book has a great central conceit. On Earth, gravity varies a small amount, but is roughly constant. On Mesklin, the planet featured in "Mission of Gravity", gravity varies from around 3 times Earth gravity at its equator to hundreds of times Earth gravity at the poles.
We experience the world primarily through the eyes of the crab-like Mesklinites enlisted by their Earth allies to complete the titular mission. Through their experiences, Clement does a great job of fleshing out this world and the limits imposed. Plants, animals, and societies are adapted to the local conditions, and as the conditions vary, so do each of these.
Although the Mesklinites are not as alien in temperament as we might expect, they are not portrayed as savages or cannon fodder (as, for example, the aliens in "Jem"). They are canny partners who are less technically advanced than Earth men, but who understand that their ability to operate unaided on the planet gives them leverage.
The characters and narrative of "Mission of Gravity" are solid, but the greatness of this book rests on the strength of its central conceit and the detail with which it's brought to life. Highly recommended.