Serviceable game but nothing really stands out about it. The titular Impossible Lair is a fun idea in theory but I found it - especially the boss fight parts of it - a bit tedious in practice.
The regular levels don't really have a lot of character, I found them mostly...unremarkable. You play through a level and nothing really sticks out, neither good nor bad. It's just a thing you did. There aren't really any creative twists on it (even in the altered versions of the levels) that make them stand out from the rest. This is not a bad game, it's just never feels like it reaches what it promises to be.

A for the most part enjoyable if mostly unremarkable game. The idea of using magnets is neat in theory, in practice it lead to moments where I wasn't sure if the solution I came up with didn't work because I was wrong or because the physics went haywire and while it usually turned out to be my fault, sometimes it simply worked after 2-3 tries, which is not ideal for a game like this. As it turns out I also don't know how "fucking magnets" work. Also while it is absolutely necessary to show the player the radius of the "magnet field" in the more busy levels later on this can often lead to a mess of colors everywhere where it's hard to decipher what's actually going on. This can be solved by simply demagnetizing every object you don't need but I'm lazy!
As far as the story goes, listen. I'm not a huge Lovecraft fan, right? But even I feel like the setting deserves more than your character going "AH! A big fish!!" or some random side characters you really, really have no reason to care about going "ajksdjalkf I'm going crazy!" every couple of levels. At least the ending, where it turned out that all old Cthulhu ever wanted was friend was a nice surprise.
At the end of the day, there are better Cthulhu games out there (some even made by Frogware themselves!) and there are better puzzle games out there. If you have already played the Portals and Talos Principles and Antichambers of the world sure, give this a go. Just don't expect too much.