The apps available in the App Store are quite good. I have both GV Mobile+ and GV Connect as well as the official free Google Voice app. I prefer GV Mobile+ out of all 3 of them personally - it has a nice interface and has been reliable though setting up push notifications takes a little effort and is not automatic like it is with the official app.
I don't – I no longer have the official Google Voice app on my iPhone but I still have both GV Mobile+ and GV Connect on it. Part of my day job is testing iPhone apps and I like to keep an eye on the updates made to both of these apps.