I've been using google voice with an android phone for the last couple years. It was nice, but I really hated my phone. Glitchy, low battery life, other issues, etc. The only good thing was google voice.
I didn't want to get an iphone because the compatibility of the google voice app wasn't as good as with an android.
When I lost my phone last week while traveling, I was kind of relieved, so i decided it was time for a change. I went to sprint and got a new iphone 4S. The phone is about 1000 times better than the android phone I had.
When I went to set up google voice on it, I found out that with sprint doing a couple clicks on google, I don't even need an app with the iphone. Its nice because I sync my google contacts and get to use the built in phone and message app and everything automatically comes from my google voice number. No google voice app needed.
Works better than android. Love it.