The Good:
I've always enjoyed Cyan's work, both in terms of puzzle design and environmental place-making, and there are definitely good examples of both those with Firmament. The visual design of the world is compelling with lots of intricate detail in architecture and mechanisms which support the story and environmental puzzles.
The Bad:
While the visual design is a strength, the execution of the environmental puzzles within the context of the game engine left something to be desired for me. Occasionally, the puzzle execution felt fiddly, requiring the player to precisely line up contraptions with convoluted control schemes in order to execute an action. The challenge should be in the mental task of figuring out the puzzle, not in the mechanical/manual execution of the task once you already know what the solution is. Independent of the puzzles, themselves, this feels like poor design in usability, which made the experience a bit rougher than it could have been.
This isn't a huge knock because the visual design is good, but it is apparent that Cyan's development/art team is small. There isn't as much visual diversity between the realms and some of the primary structures you traverse in the game are duplicated between realms. The snow realm felt the most unique/compelling.
The Ugly:
I could live with the above and still consider this a good game that I'd recommend, but there were two instances of game-breaking bugs that soured my experience. Both were collision-related. The first was getting stuck in a gap between a couple manually aligned platforms. When reloading a saved game in this state, my player would fall through the crack down through the world geometry to a hidden ground plane. This required backtracking to an older save which was annoying but not too time consuming. The second issue was also a collision issue that occurred while driving the spider walker around. At one point, I moved between the top platform and the interior of the walker via the ladder and discovered I was pinned to one side of the space by some invisible collision (which I couldn't associate with any visible geometry in the world as I was in a somewhat open area). I could still squeeze around enough to move up and down on the ladder but was blocked off from getting to any other controls on the walker. This was the last straw. While I could reload a save an not have lost too much time, I was worried that I would experience more issues like this if I kept playing.
Summary:
Good visual/environment design (despite some repetition) and decent puzzle design. Some fiddly puzzle execution within the context of the game engine and collision systems, resulting in collision-related bugs that broke the game for me on multiple occasions, leading to frustration. I want to recommend this game as I enjoy Cyan's work and I know they have been supporting the game and working on fixes, but I cannot at this time given the game-breaking collision issues. I hope they resolve more of these and I'm able to return and complete the game at some point.