The Secret World has done an amazing job utilizing voice work into an MMO. Your character never speaks. Cutscenes are always observing conversations between NPCs or from an NPC to you, not you speaking back.
It does bring the world to life in a way that MMOs could use to attract the younger generation of players. That being said, it should be used sparingly, and a mix of voice and text is the best approach, in my opinion.
So for example:
NPC 1 (V.O.): "So if the Trolls aren't put down fast, Superadine is going to flood the streets. I don't even want to imagine what Skyway City will look like after that happens. Are you in?"
Your character: select one of the following text box responses
[I'm in. Let's move.]
X[I agree, but I have something I have to first. I'll be back, I promise]
[I'm sorry, but this mission isn't for me. Good luck and godspeed.]
x- represents your selection
NPC 1: (V.O.) "OK, but please don't take too long. Those shipments could start hitting the streets any time now."
Also, @Irish Girl: You are amazing. That work you've done is incredible. Not only does it REALLY feel like CoH, it looks even better! I'll be in touch to help once you settle on a way to contact you and begin organizing. I'll be bringing the CS6 suite and my own UE4 to the cause. :-)