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I think the main game is very good and the major DLC (Road to Gehenna) makes it great.
On the surface it is a puzzle game (put weight on pressure plate to open door, find ways to link light source with light-sensitive switch etc), with beautiful graphics and an interesting setting. Once you get past the fact that the game explains almost nothing, the learning curve is shallow, with only some of the challenges being really tough, but the learning process is also quite fun.
Some people will stall right at the beginning of the game and have to go find walk-throughs, because the game really does explain almost nothing. You have to explore to find out what the challenges even look like, how you'll know when you've finished them and what to do next (a bit like the Myst games). If you're not basically skilled at puzzle games, I guess the learning curve is actually very steep.
There's also an interesting conversation going on about about philosophy and how to find meaning in life. You can ignore that or treat it as just an interesting backstory - and that bit is also full of good jokes, so it's not boring. But in fact this is a game that uses the mechanics both to make satirical jokes about computer games and to demonstrate, brutally, the importance of the issues being debated (a bit like Bioshock satirised FPS games and also had an interesting conversation about Ayn Rand's wacky philosophy). There are important choices you can make in the game and the philosophical debate can help you think about those choices - and vice versa.
In the main game, the challenges mostly aren't too tough and and I wasn't new to the philosophical debate. I enjoyed it but was more thrilled by how cleverly it has been designed to coax the typical gamer into thinking about those things while still being fun.
Then I played the DLC, which turned all the dials up to 11. Much tougher challenges, no hints, harder choices to make in the game, a whole new kind of minigame, lots more good jokes, a whole extra dimension to the philosophical debate.
Both the main game and the DLC have multiple possible endings. I enjoyed the game so much that I spent quite a bit of time chasing down the bonus puzzles before playing the toughest (and most emotionally satisfying) endings, because I knew I was going to miss playing.
I had to edit this review a lot. I didn't have any actual spoilers, but discovering what kind of a game you're actually playing is part of what makes it a great game, which I don't want to spoil for anybody who likes that kind of thing.
If anybody was curious about Mosaic, let me spare you some time and money and tell you it's pretty shit. Marketed as one of those slow-burn games that has a serious and complicated message behind apparently boring and repetitive game play (something like Stanley Parable, Death Stranding or The Talos Principle)), it's actually a very linear interactive novel. And not very interactive. And not very interesting.
I had to make myself stop playing The Talos Principle, especially once I got onto the DLC. Had to make myself finish Mosaic and now I want the time and money back.