I absolutely agree that Kant's system as represented in the Groundwork is unworkable. But you could say the same about pretty much any pre-20th-century philosopher's major work. I think the fact that someone was even trying to think about ethics along essentially game-theoretic lines in the 18th century is pretty revolutionary and worthy of respect, even if he did get important things wrong. As far as I'm aware, no one else was even in the ballpark.

ETA: I do think a lot of philosophers scoff (correctly) at Kant's object-level moral views, not only because of their absurdity (the horrified tone in which he describes masturbation still makes me chuckle) but because of the intellectual contortions he would go through to "prove" them using his system. While I believe he made very important contributions to meta-ethics, his framework was nowhere near precise enough to generate a workable applied ethics. So yeah, Kant's actual ethical positions are pretty scoff-worthy, but the insight driving his moral framework is not.