I read the whole trilogy as a teenager. I got all three in one volume from the Science Fiction Book Club, and I remember reading it during a snow storm (ideal, given the subject) so maybe winter of 1977 when I was 18.

Forty years later, I can remember quite a bit of it, so I suppose that says a lot for its memorability. Much of the fun of the trilogy was exploring the world, and there genuinely were interesting things in each new location. I definitely liked the action/adventure aspect. However, what I remember most was how much the disregard for scientific accuracy and the overt political message annoyed me.

Ostensibly, this one starts off as a straight SF story, with the hero visiting a planet that is suffering because its star is getting dimmer for some reason. When I learned the main character had been raised on Mercury by natives I almost abandoned the book. I wasn't happy when he found the people who lived underground on Skaith and it turned out they had magic. They shouldn't have cared about the dimming of the star, but apparently this was causing the planet's internal heat supply to cool as well. Gaah!

The political message, that if you give people welfare, they'll feel entitled and ungrateful, they'll control your government and eventually they'll destroy everything, was really heavy-handed. I'd grown up in a conservative Southern family who largely agreed with that message, and, at that age, I hadn't rejected it yet, so I wasn't upset because I thought the message was wrong, just that the delivery was lame.

Now one can argue that this was a future fantasy (I like "Planetary Fantasy" as a subgenre, though), not an SF story, but that's a genre that's hard to pull off, even today. This one is pretty much a sword-and-sorcery story in SF drag. I'd probably be more willing to forgive that if I read it today. The message fiction, not so much.

I think your comparison with "The Dispossessed" is spot on; I read that book the summer before and dearly loved it. I reread it a year or two ago, and found it even better than I'd remembered it.

Upshot: I did finish all three Skaith books, but I was never willing to ready anything else by Leigh Brackett.