I use it all the time both on my mobile phone as well as on my desktop. Let's face it, when you get older you can't keep up with the kids when it comes to texting, but give me a keyboard and google voice and I can out text them any day.
I also use it to make calls from my computer and mobile and find the quality to be very good.
I just finished building a PBX in a flash server and added my Google Voice account as a Trunk to the server.
I'd put this on your top ten, but then that would be eleven, mabe you could link it to google talk since they are linked anyway. Or maybe that's what you meant when you listed Google Talk?