Tons of pro and con discussion on this film in these two threads:

http://monsterkidclassichorrorforum.yuku.com/topic/26742/An-American-Werewolf-In-London?page=1 

http://monsterkidclassichorrorforum.yuku.com/topic/4566/THE-HOWLING-vs-AMERICAN-WEREWOLF?page=1 

I've loved this film ever since I saw it back in 1981 in the theater. All my comments are in the above threads.