My G4 works quite well with my Blackberry Storm, which I use to play music (via the built-in mp3 player) as well as to make or receive calls.
If I receive a call my while listening to music, the music will mute and I will hear a jingle. Although I can answer the call by speaking loudly (practically shouting), I find it easier to just reach up and push the top middle button. When the other party hangs up, the music will resume playing.
If I want to make a call, I push the top middle button to activate the voice calling on my BB. I can then just speak the name of the party and it will dial automatically. When the call ends, my music resumes.
Sometimes the music will cut out or fail to resume after a call. I can usually re-start the music if I hold the front side button for 3-4 seconds.
When my wife is riding with me, I typically leave the VOX open and do not use the music player on my BB. As others have noted, voice activating the system can be difficult.
Sometimes while I am connected to my wife via VOX, the unit will start to pick-up static. I can usually eliminate the static by turning the VOX off and then back on.