Me and a friend just tried CM explore mode...
it took us 3 hours and over 40 deaths to complete. I feel that it took a really long time, but that fact is offset by the fact that you are awarded with medals for your work. (we got 25)
These medals, each uniquely awarded for completion of each dungeon, can be turned in by the hundreds to redeem rewards in the form of exotic armors, sigils, items, and weapons. I am going to look further into this as I study the dungeon system.
We also completed the Citadel of Flames story dungeon (level 70) which wasn't very hard. It had neat rewards, typical of story dungeons, like the little jolt of experience and the rare hat.
It was also pleasantly fun rather than monotonously grinding. I recommend this dungeon.