While leveling in Northrend, I built up this
spec from 70-80. It afforded me little downtime since I just kept moving and DoTing up mobs while my Felguard cleaned them up. The Siphon Life on multiple mobs allowed me to Life Tap and keep high levels of health and mana. I was also able to solo most group quest mobs in Northrend using this build.
This was more suited toward leveling and killing groups of mobs quickly and efficiently while maintaining a high level of survivability. Being level 80 and with better gear than quest rewards, you can probably tweak this build a bit. For instance, you probably won't need 3/3 Suppression, so you could put those points somewhere else.
Just remember to keep Omen on and watch your aggro versus your Felguard. This means usually you will be just refreshing DoTs on the mob, with perhaps the occasional Shadowbolt. Also, put down a
Demonic Circle somewhere safe before you pull so you can teleport away quickly if need be.