This is part of my reading of past winners of the Tiptree (and Clarke and BSFA) Award; I must say I had not heard of either book other than in this context; I was pleasantly surprised.
Walk to the End of the World, the first half (I got the combined edition, The Slave and the Free) is a horribly well-drawn future dystopia where women are enslaved and brainwashed, and doped up men fight for their own continued supremacy. It's gruesomely well depicted, though not at all subtle and a bit relentless.
But Motherlines takes a lot of Walk to the End of the World and inverts it - we switch from a male to a female central character, and discover that a lot of what had been presented as unchallengeable fact in the first volume is in fact very different looked at from the other side of the gender divide. In addition, the actual plot has some very impressive twists and turns in what is still a very short book.
Motherlines is really excellent, and though Walk to the End of the World is not quite as good you enjoy the second much more for having read the first. And neither is very long.