Challenging for the most part. Except for when it comes to the Power Plant Mission that mission is damn near impossible on Challenge. Now maybe I'm just a noob. But why is the other Challenge missions not as hard as the Power Plant one? I mean everyone needs to be geared out just the right way. And I myself only have 4 High-End pieces. Just something I've noticed that for some strange reason that mission is so hard on challenge. Even when I went through it for the first time on hard it was challenging. Never played a mission that was so frustrating that it actually made me want to give up playing the game for a bit and come back.