The best way to use DC in my experience (and how I use them) is to take 8-10 in a storm raven with Chappy/Reclusiarch/Lemartes with a Fist, and 2-3 PWs. A LR can be an alternative, but I prefer the speed of the Raven (also see a lot of dark lances and meltas anyway). You definitely need a transport as a smart player will counter them too easily with rage.
While relentless bolters seem like a good idea, they tend not to be in practice for DC. Using bolters means you plan to walk for a round of two. As such the enemy can rage kite you. Also, if you are close enough to shoot and assault, you are MUCH better off shooting pistols and assaulting with the extra attacks. While mathematically, it is the same total amount of possible wounds (pistols+extra attack = bolters+less attacks) you have to consider the fact that chappys give them rerolls to hits (WS5 means hitting on 3s usuall) AND wounds (FC means you wound T4 on 3s as well) in combat. This gives you a much higher potential of actual wounds in CC than you do with bolters.
I suggest a few PWs and a Fist to make sure they get some punch. I'll stick the special weapons on an IC to make sure they feel some heat. Fist helps with Dreads, MCs, and in case you get stuck raging towards a vehicle at some point.
People get a little confused when they see DC in the troops section. Just because they use that FOC, I do not treat them as such. To me, they are an "elite" unit and should be used in the same way you would use other supporting units. I tend to use them to spear head for my list and absorb fire, damage, and attention. Rip up and weaken as much as they can as my assault squads move in behind to clean up and claim objectives.