The only enrage i've ever hit in HM flashpoints is Storm Squad at the end of boarding party which is already known for being overtuned. That and Soa, the last boss in the Eternity Vault.
They give pretty ample time to kill things considering the group I run with is using blue 50 - epic 52 gear.
Depends on group comp. Any group with a sniper/gunslinger will beat enrage timers handily, even with a badly-geared sniper. They put out far and above more DPS than any other AC right now.
Unfortunately my guild doesn't have one yet.