Well, my own experiences are mostly positive, but I only do voice chat with people I at least loosely know beforehand. Otherwise, even an integrated voice chat is off by default for me and I personally almost never speak myself - too much hate for my own voice.

It's useful to save typing time and it's almost required if you want to coordinate something while the fight is running, since you need your hands free. It's also useful for callouts, since you sometimes forget what was coming next or tunnel vision. In a good group that knows the fight in and out, it doesn't really seem necessary however.

And that's pretty much it.