Just finished it. It gets more engaging later, but in a bad way. I was bored for the first ~2 hours of the game and could barely bring myself to slog through it, and then I was frustrated through the second half when the puzzle solving took a back seat to the platforming, feathering objects around with a hair trigger on the antigravity, and lots of annoying conveyor belt puzzles. There are practically no challenging puzzles, but I suppose if you like first person platforming with some mechanics, this might be the game for you.
There was only one puzzle in the game that I found actually challenging as a puzzle and it was the one where you choose one of the four IDS batteries and have to acquire the other 3 in order afterwards. It was a clever little gimmick, but poorly implemented. I could not solve it when I started with the antigravity battery, but it was literally trivial when I started with the time slow. Time slowing immediately let me get the light dimension from behind the fan, which let me laboriously block crates for about 5 minutes to get the antigravity battery and then the heavy battery.
Most of the time, the the puzzles feel like rehashes of previous puzzles and the only challenge I found in most of them was in the execution of the awkward platforming and using the right dimensional shifts at the right time. When you start mixing 3-4 shifts in harmony alongside flying on a couch through lasers and low ceilings, you've left your puzzle game in the dust for a pure platformer, and I fucking hate platformers.
For a game so rich with mechanics, both in the hands of the player AND in the hands of the AI, the game never felt really clever to me. There's all kinds of stuff happening in the game compared to something like Portal, but it ends up feeling cluttered and unimaginative rather than clever and interesting. Where Portal 2 has about 7 "things" that all feel nicely distinct (lasers, light bridges, excursion funnels, faith plates, and the 3 paints) QC just feels like an arbitrary mishmash of random shit happening.
Portal is beautiful in its simplicity, and to this day map makers are putting up maps that do new and unique things with a small set of mechanics. Those kinds of puzzles feel great to figure out because when you get stumped on them, you feel like they've legitimately made a hard puzzle rather than just inventing some new rules. Once the real game starts about halfway through, QC feels like cluttered by comparison, with at any time a whole bunch of cheesy different ways that I could approach each puzzle and do it successfully.
It's a subtle thing that I feel about it and I'm not sure if I'm explaining it properly, but so many of the mechanics felt contrived later on. Like, we want to have the player do something, so rather than cleverly using stuff we already implemented, we'll just add some weird new thing (like the robots that rushed forward in a region to knock blocks over the edge out of your reach). The stuff that they reuse is always the same schtick, while Portal does such cool things with such a small subset of tricks up its sleeve.
I'm not even sure what happened at the end, and quite frankly I don't care. At that point I was frustrated and sick of the game and just wanted to finish it because I spent the money on it. I wasn't intrigued by what was happening and I was sick of John de Lancie babbling incoherent, unfunny nonsense into my ear.