I have my cell phone's voicemail use Google Voice and it's a huge improvement over T-Mobile's voicemail.

I also give out my Google Voice number and it rings both my cell phone and my always-open Gmail window at the same time.

If I'm at my computer with headphones on, which is most of the time, then I can make/receive calls for free inside of my web browser. I know people want to use GV for more mobile things, but I'm extremely happy with the way I'm currently using it through Gmail.