If you start hellfire quests at 58, be prepared to instance grind before you can go to Zangarmarsh -- you won't be high enough level for the level prerequisites in the next area.

People will also tell you that Nazan in HFP is a pain in the rear.

Keep that in mind when making your decision. I waited until I was 60 to go to hellfire, and since most of the pre-BC level 58-60 areas are completely deserted, I had no problems finding quests.