Really lovely. As a lot of people have said here it feels like playing Doom for the first time, the difficulty being similar - just substituting the awkward old tank controls back then with the ninja WSAD abilities we have in modern times.
My only complaint would be that the secrets are a bit obnoxious to find but this could be a nod to the original Doom so I'm willing to let this go. And the clumsy textures on E1M1 almost put me off but I'm sure glad I persevered.