3D doesn't add a huge amount, but the conversion is done very well so it does look great in 3D. If you have an extra couple of dollars, see it in 3D for the experience, but if you see it in 2D you aren't missing anything.
I work in a movie theater and we're having a premiere tomorrow night for this. As part of the employee privileges, I get to see it in 3D IMAX for free with my boyfriend. My manager got me a movie poster as a gift since she knows I love the X-Men so much.