I'm kinda new to the healing game. I was a shadow priest to 62, but I'm now holy. I've only healed 3 instances. Last night I pugged a group that had two warlocks. I've never had much to do with a warlock, and I was watching their health bars go up and down. I'd keep putting renew up, and every now and then I'd throw in a flash heal. (Our tank was well geared and I had plenty of mana.)
After a while I got a message from one of them letting me know they were fine with a renew and they didn't need any other healing.
As I understand it, warlocks convert health to mana, then drain life from mobs to get their health back up, but it freaks me out as a healer to see bars drop to 50% or lower.
I'd love to know a warlocks perspective about what kind of healing you want / need in an instance.