I'm up in the air, I'm thinking about buying it tomorrow, but I just bout L4D2, so I don't know if I want to spend the money on this too. If I do, I'll definently play with you.
I'd recommend Teamspeak or Ventrilo.
The games' ingame-voicechat is horrible. It's voice activation and you need to be literally swallowing the mic or yelling your heart out for it to pick your voice up.