I chose half-and-half, although I wouldn't have chosen the term "adversary". As DaveMage said, the DM as adversary is an insta-win (and then very lonely). At the same time, it is my job as DM to make the players and their characters work for it. Easy fights, free treasure, unearned rewards (including non-mechanical IC rewards) tend, IME, to bore the living crap out of most players. It's a game, first and foremost, and games are more fun when they are challenging. That's true for me as DM too -- I am having the most fun when my players are chalenging me: overcoming my challenges, "winning" the adventures, pushing my game into unexpected directions, and so on. And it is true of the power gamer ("That combo does WHAT? -- Good show!") and the immersive role-player ("You do what with the queen? -- Good show!") alike.