The major issue we have is the fact that text messages aren't "intercepted" by the Google Voice app in order to send it through your GV number. You have to send your messages using GV instead of with the default messaging client. This wouldn't even be an issue (the unofficial iPhone Google Voice clients have this as well), except that outgoing calls DO get intercepted and piped through Google Voice. The inconsistency is what's bugging us, and since this is Google's own OS and Google's own app, there should be a way for them to implement this feature.
Other than that, everything works pretty much as expected, and better than other GV implementations on other phones. You can view all the folders in your account, listen to voicemails, see their transcripts, make international calls and do just about everything you can do on the Google Voice webapp.
It's free, so get it now.
Texting (inside the Google Voice app)
What the app's allowed to touch on your phone
The folders you can view
Account balance (for international calls)