I'm an immense fan of the first two, but I didn't like Requiem at all.
Remember at the end of Black Plague when-
Don't get me wrong, I really, REALLY liked Black Plague. Especially the actual end that came after that part. But I didn't like the gameplay in that particular segment, and Requiem just felt like 4 hours of that. It's really all over the place and feels more like a fan-made mod for Black Plague.
Still, I can't express how much I like everything else the guys at Frictional have done. Requiem is literally the only bad thing remotely connected to them and Machine for Pigs is fantastic so far.
I'd love to hear your thoughts on Requiem once you play it.