Succubus and felhunter does not suck, but they are situational. I've been using the succubus more frequently lately, because of the damage output and seduce ability, I'm killing much faster with her than with big blue. The felhunter has its uses too, just try fighting casters with him and you'll see.
As for talents; hard question. Myself I've been going all affliction and am nearing dark pact, which will mean highly increased kill speed due to a "neverending" mana supply. Destruction is good as well, but you can't really get both as high as you want
One tip regarding gear though: Go for +stamina gear, sinece we've got life tap we can use our health bar as mana pool so no need to max mana in the same way. In addition you will find you can tank most mobs yourself with a high health amount. Right now at lvl 38 I have 1800+ health and 2300+ mana and find that I stay alive for very long using drain life.