All of my players are also DMs, and specifically have DM'd me either a lot or at least once. For us, the illusion of "Rrarrr! I'm the adversary!! Wink-wink!" doesn't fly because we know all of the tricks of the trade. It's like designing a funhouse for guys who design funhouses for a living. We admire a good solid design, we ooh and ahh over something innovative or different, and we sympathize when something doesn't work out as well as planned.
I get the greatest pleasure, as DM, out of setting up a challenge and seeing the players stretch their tactical thinking, use their available resources (including things like environment/terrain) to their advantage, make gutsy choices, and utlimately I like to see them victorious. Doesn't mean I hand it to them. But I'm definitely rooting for them and excited when they succeed. I'm certainly not interested in beating/defeating them -- that's not part of my DM vocabulary.
