I don't have the GV app installed on my phone, but I've had a number for the past 2+ years (from when they were still Grand Central), and all I can say is that this service is freaking awesome. When Google took over they added some nice features.
Listen to your VMs anywhere (phone, email, web). VMs get converted to text & emailed to you. Have your phone ring through to anywhere you are (home, work, cell, or all of them). Group your contacts, just as you do with email (all work contacts ring at the office, all friends/family ring at cell, etc). Call or SMS from your GV number. Custom VM intros for each number or group of numbers.
I haven't switched to GV exclusively, but I'm considering it. I've had my cell number for 10+ years, and don't really want to give that number up. I use this as a 2nd phone for people that I may never want to talk to again, such as buying/selling on Craigslist/eBay, calling to find out information about products, etc. Its also come in handy for times when I have sketchy cell coverage but can find wifi.
Honestly I'm kind of surprised that its free. If Verizon had this it'd be $30/mo.