If you're reading philosophy for brilliant and engaging prose, you're going to be disappointed time and again!
The thing with Kant is that what he wrote is widely accepted as a foundation for contemporary metaphysics and morality. Mind you,
Critique of Pure Reason wasn't well received even in his own time. I haven't read it, but I do know the basic tenets of the work.
If you familiarize yourself with Kant's teachings, whether you read him directly or not, you will better understand what Nietzsche and existentialists are getting at.
If you don't know the target of the critique, you won't have an easy time getting your head around the critique itself.
I think the challenge here is that we're attracted to writings that are deconstructions and criticisms moreso than we are to writings that establish a comprehensive understanding of metaphysics at its core. This is why it's difficult to read Kant. I don't think anyone finds him an easy read, but that doesn't mean we shouldn't make the effort to understand what he's saying.