Because the Succubus is so fragile, the vanish works to keep her alive just a little while longer. It's excellent for PvP despite the fact that she'll really hardly get stunned or snared. However, the free movement after vanish allows you to reposition her better. Note that it is an invisibility effect, and not stealth, although I haven't tested it extensively to see the difference.
This is great for the times when you need the big blue to tank for you. The problem with it now is that it simply doesn't generate enough threat and although I would like to see that problem fixed baseline, the added threat from Demonic Empowerment coupled with Torment or Suffering works extremely well. The health buff is alright, too, specially for tanking multiple mobs.
Too bad it's impossible to get this talent and Empowered Imp deep in Destruction. For good reason, too, since it would probably be obscenely strong. On the other hand, a 20% increase to the imp's crits every 30 seconds is phenomenal, even if you just use the imp occasionally. It's a definite rise in DPS and is synergistic with Demonic Pact and Demonic Empathy.
This is the one effect I'm not too happy about, although it works thematically well with the Felhunter. It brings it back to its roots. The Felhunter is an anti-caster pet. It only makes sense that its uh-oh button removes magical effects because it's more than likely that you'll be sending it against a Mage, another Warlock, or even a healer. This effect removes snares, debuffs, fears, and theoretically even a Banish effect even though I haven't tested it. Your puppy will still die to Warriors and Rogues, however...
Because a Demonologist is highly likely to have the Fel Guard out, this is perhaps the best effect of all. In PvE, the haste is a definite DPS boost (although I would've preferred crit), and the immunity is awesome in PvP. It's almost the equivalent of Bestial Wrath. You'll be using this almost every minute in either situation, although more judiciously in PvP.
Overall not a bad spell, right? Sure, some of the effects could use a few buffs, but to be honest I'm pretty happy with the spell conceptually. It's one talent point for a utility spell that makes our demons more useful (or saves their butt) every minute. Every Demonology Warlock should take it, particularly because there aren't too many active abilities in Demonology.
It's a utility spell that adapts to the situation depending on the pet you have out. The effects make sense, even if they aren't overwhelmingly strong. I think that if you look at all the effects all together, it's pretty powerful. But if you realize that it's only usable by one pet at a time, then it's slightly underpowered for a 31-point talent. Still, it's one of my favorite talents in Demonology right now and once you've learned to use it with your pet, you'll realize it's a talent point well spent.