Depends on the game. In WoW, for example, while it does have some fans, it's a pretty negative feature overall- it was very much shoehorned in and often has a bad influence on game balance that damages the rest of the game. However, if we're talking about something with more meaningful PvP, namely a game featuring RvR content (like Dark Age of Camelot, Elder Scrolls Online, or to someone extent GW2)or an immersive sandbox (for upcoming games, Age of Wushu or Pathfinder Online), then it could be a great thing- it really needs to be a core focus of the game and far more meaningful than WoW. Stuff like team deathmatches are never good. And if PvP is completely separate from the rest of the game, it has to have it's own balance! Anything else would be very harmful to the game, like in WoW.