What people says it's that flying feels great. And I agree with that.
But that's the surface-level analysis most people do. How things feel. Gameplay-wise, I think the flying system is a bit wasted, it isn't as tightly integrated into the gameplay like it could have been done. Say, having parts of combat where you also fight while flying (and not just hovering), maybe against enemy freelancers? Or have more enemies with energy based weapons that can be avoided (I mean, like a fireball from an Imp in Doom), and having enemies with more special attacks which can be avoided, making movement more important.
That's the thing though, the flight is just for traversal for the most part(and sucks on the KB+M). Ironman integrates flying and using the resultant speed and momentum extensively into his arsenal so I don't get that comparison either beyond exosuit and boom boom.