There are good and bad effects of all kinds, and there's also the ethos of "special effects are just a tool, a means of telling a story." (Can't quite remember who said that :) )
I'm still amazed at how much I "buy" the character of Gollum; I can't "buy" the Na'vi in the same way. And for all the examples Boost mentioned, you have King Kong, or the original 1954 Godzilla, the original Alien, Carpenter's The Thing, Cronenberg's The Fly...all *good* stories.