I've been hopping around on slight variations of shadow and Disc/Holy specs since I made 58. My Performance in Hellfire has not been to my liking; I switched to a spec that claimed decent DPS and healing, but I'm going to guess that what it meant was "decent healing and existent DPS". No, I was not happy.
So I switched back to pure shadow, but that, too, was not making me happy. The DPS was OK, I guess - around 220 at level 60 - but I had absolutely no points going to healing talents at all, my mana pool was shallow, and my mana regen rate stank on ice.
So, I tried a new spec, going deep into the Discipline (Why do I always hear the sound of a whip cracking?) tree, with some bits in the shadow and holy trees to enhance holy and shadow damage.
Result: DPS is lower (around 215 at level 61) but I hardly ever have to stop for mana, and my survivability is extremely high. I like how BRK once described the D tree: "cockroaches". The keys to this survivability are the double whammy of a hardened PW:S and an improved PW:S that reflects damage back on the attacker - including ranged physical and magical attacks.
Meanwhile, Hellfire has been good to me. Aside from one hyperactive git of a Palli, there have been few serious issues that I could not contend with. I just finished up the demon quests in the southeast and got my wings - and this hat. Stylin'!