The main thing this comp lacks for heroics is CC. I initially tried heroics with a 3 frost, 1 blood setup but found the frosties pulling agro in 4-5 pulls and dying very quickly. When they didn't and my PvP geared tank received 4-5 x focus, my PvP geared priest just could not heal him through it.

So I switched to blood in patch 4.0.6 for heroics with the buff to death strike and heart strike and lovely rune strike spam. I use exactly the same ISboxer setup for my blood spec, but my dps bar just contains death strike, heart strike, rune strike, instead of howling blast, necrotic strike, frost strike etc.

What I get from this is 4 guys who do 6-7k single target dps, going up to 10-12k dps when pulling groups. The self healing is absolutely insane. The first time I did heroic Rajh, my priest died to the charge thing at about 95% but between AMS, IBF and the self heals, the 4 DKs downed him without dying. Admittedly this was due to a fairly lucky run of casts from Rajh without a charge at the group but it still shows the potential. They all ended up without about 150k of healing each from memory.

Now I haven't attempted all heroics with these guys, but I can't imagine trash giving me a problem. The only place that was slightly annoying was Vortex where I ran into a few dual healer pulls at the end. Once I worked out these guys had virtually endless mana and 200k heals though, a couple of well timed strangulates did the trick.

The two places I have failed (stopping me from trying the rest of the instance :P) were first boss in Grim Batol and second boss in BRC. In GB my heartstrike spam would eventually kill the purple guy and I was only able to keep the team up until about 800k left before dying. Replacing heart strike with blood strike reduced my dps to 5k on the boss which just wasn't enough to kill him before the damage from adds etc. became overwhelming.

BRC was really just me not being setup to block the rays well. I thought I could waltz in and just kill the boss through 3 enrages. When I tried blocking with the priest I mistimed it and got transformed and decided to call it quits.

Haven't played much the last fortnight, getting my shaman team set up to PvE a bit more instead as I feel that Blizz will continue to make the CC part of instances necessary throughout the whole expansion. This is what cripples all DK teams in reality, the only thing that lets Bloods get through it is their insane survivability.