I think its both better and worse depending on which aspect your talking about. First the 3d adds depth to the game play that gives you a more immersive experience. There is a extra level of tension as you feel more connected to the game, especially in the 3d zombie mode. The 3d looks spectacular especially when aiming down the sight of a gun. At first its harder aiming down site because of the extra landscape and depth you are given in 3d. 2d mode aiming is more linear, you just line up a dot with an enemy, but in 3d it feels like you have to compensate for that extra dimension also. After a couple of hours playing i got use to the 3d and it actually feels that I have a slight advantage in aiming compared to 2d. And this shows in my kill/death ratio, all the other COD ive played my k/d was @ 0.85, now in 3d I'm 1.30.
But on the other hand the 3d is not the best representation I've seen in 3d gaming. The 3d is milder than I would of liked but maybe with the fast paced game play stronger 3d might not have worked as well, but at least give the option to adjust the 3d strength. There is mostly just depth added to the graphics, except for the popping out of the sights. I wish there were more popping out of screen 3d, explosion shrapnel coming at me or zombies sticking their arm out of the screen to get me would have been sweet. There is also a noticeable hit in the graphics when placed in 3d. The graphics looked slightly grainy with rougher edges, although this is slight its not the smooth edged graphics of the COD i'm used to. I havent noticed any frame rate issues when playing online multiplayer, but i did notice one instance of slow frame rate in campain mode where there was a ton of action going on at the same time.
All in all i enjoy the 3d mode which i continue to play in, but I think Treyarch took the safe route and the 3d could have been better. Also the 3D was an add-on late in the game's developement, I cant wait to see a FPS with 3d in mind from ground up.