Downed heroic halfus 25 last night. Was a very smooth attempt, finished with everyone alive. Getting these fights down isn't about mana (although mana is important, managing the fight is more important). It's about management and control. You have to set the fight to go the way you want it to, not meet the fight on its terms.
We ran last night with 3 holy paladins, 1 shaman, 2 disc priests, 1 druid. Our usual composition is 2 holy paladins, 1 holy priest, 2 druids, 2 shamans. Unfortunately we are losing one of our shaman to new-daddy issues. Since we just got a couple of new holy paladin trials through we've passed one as a probationary and he subbed in for the kill.
Getting it requires competence, not perfection, but competence across the raid. The fights are fun, challenging, unforgiving, and make it worth every minute.