holy fuck challenge modes are hard as fuck. my mage is 491 and a friend decided to start them, we went in expecting a little difficulty but just the trash hits like mack trucks. After 2 hours ended up getting silver in SM with a fuck-ton of wipes, you literally have to learn every single inch and every single pull to minimize time and damage, while still killing the required number of mobs, and pretty much will need a fixed 5 man group.
Nice thing is no durability loss, and you can reset the CM anytime but yeah, if you want challenging 5 man content this just blows away anything else I've seen in a long time including GW2 fractals, TERA hardmodes, Rift heroic challenges, etc. Nice thing too is since they scale everything down to 463 ilevel, it should stay difficult all throughout MOP.
Definitely not a thing for me, I'd rather just chill and play easy than be focused for hours, but if you can't raid and still want really, really hard content challenge mode is built for you. Be interesting to see if they change them at all during the life of MOP, either to be more accessible or to give more rewards.