Vampires in traditional folklore are the incarnation of evil, and are usually seen as "not nice" by definition. However there have been authors who successfully make vampires, if not "nice", then at least sympathetic. However, while they may be sympathetic, or may be on the main characters side, vampires should still have an element of scariness, of badassery, and they should be complicated, as befits a creature associated with the terrifying night who may live for thousands of years.
It is these characteristics that the Meyerpires lack.
The truth is, not all of them are that nice. Jasper is aloof and distant(don't forget constipated). Alice is pushy and materialistic. Rosalie is bitchy and sarcastic(of course, she's blonde ), though this does in many ways make her more likable. Carlisle makes choices for other people that he would not have made for himself, and almost always chooses wrong (the one person we know wanted to be a vampire, Bella, was the only one he refused to change.)
"BUT EDWARD IS TTLY NISE!" scream the twihards. Well...No.
What they are is universally simple. Alice is bubbly and cute. Jasper is dangerous. Emmett is strong and teases a lot. Rosalie is shallow and bitchy. Esme is motherly. Carlisle is compassionate. Edward is sexy and paranoid. The things that might have made them intriguing: superpowers, powerful passions, barely-controlled hunger, are all either nonexistant (the second two) or used only for the good of all. This isn't about being nice: it's about all of them being the next best thing to undead Miss America contestants.