While its always nice to have good cover art, its definitely not essential for me when I'm deciding what I want to buy. I care about the quality of the actual show, or movie, first and foremost.
Having really awesome cover art, though, can tend to make a DVD even more tempting for me to buy, if its something that I was already intrested in purchasing. For example, the cover art designs on those One Piece sets look really awesome, and do sort of make want to buy them. But, at $40-$50 a set, I still will not buy them, until a complete season 1 set comes out, or until they come down in price, since I honestly cannot afford them with my current budget.
Still, that cover art almost tempted my into buying one of them, when I was at a local Best Buy store, the other day.
The Yu Yu Hakusho sets, on the other hand, have extremely good prices for my liking, and I already have 1 of them, and will get the rest, no matter what they put on the cover (just as soon as I earn some more money, anyways). Of course, everyone's known what each cover looks like since before any of the sets were even released.