I think most of the opinions that the hand ruins the movie, or at least detracts from it, comes from the fact that the hand is never explained, and thus seems like a cheesy and pointless addition to the movie. In the second movie, which is based on the third Vampire Hunter D novel (both movies are based off the VHD series of Japanese novels), they finally give the explanation for why the hand is there that they never had time for in the first movie: The hand is not a natural part of D, but instead a parasite that attached itself to D before either movie takes place. In order to keep its host alive (and thus itself) when D is in danger, as well as earn its keep so D won't finally decide to just cut his own hand off to get rid of it, the parasite uses its abilities to give D an occasional "helping hand".
I don't know about you, but personally, I think it seems a little less lame and cheesy when you find out that there's an actual reason and purpose behind it. That, and the fact that myself and everyone else who has seen the new Vampire Hunter D movie (called "Vampire Hunter D 2000" or "Vampire Hunter D: Bloodlust" depending on what country you see it in) agrees that the hand is the absolute best part of the already awesome movie. Its lines in that movie are absolutely hilarious.