First off let me state that I am very happy with my Casper purchase. I originally bought the mattress, base and cover. 
While the cover seems to be of good quality and appears to fit the mattress, we have trouble with it bunching up during the night. We have always had a mattress cover, purchased in a department store, and have not had an issue with any of them. I am not sure what causes it do  that, so we just pull it taunt when making the bed in the morning.
Like I said, overall I am very happy. I hope you find this review helpful.