I was unhappy enough about the lack of Google Voice on the iPhone to go out and buy an Android phone on the spot, but I figured it was the financially prudent thing to do to see what I could do with the iPhone first. I'd heard that the jailbreak community had put out one of the now-banned AppStore apps for Grand Central (now Google Voice) called GV Mobile, so I set out to check it out. What follows are my experiences and what I think of the whole thing.
First off, I hit Google, searching for jailbreak walkthroughs and found a page that gave some very nice, step-by-step instructions. I followed these and quickly had my phone running the Cydia installer. From there, I just had to search for "GV" and installed the app. Once installed, it shows up just like any other iPhone app. Nothing surprising there.
It did crash on me the first time I used it. If you do follow my lead, I suggest exiting and re-running the app as soon as you enter your Google Voice username and password and selecting the "remember" toggle. That way, you don't have to do that part over again if there's a problem.
So, it isn't at first obvious what this app does. It's not actually dialing out. Instead, it uses the same technology that Google Voice's "Web badges" use. That is, it's going to call your cell phone and simultaneously call your requested number and connect you. That way, the caller gets your Google Voice number as the callback instead of your iPhone number. you can also initiate a call from your iPhone, but have it ring any other phone to connect (e.g. your desk or home). That's what the "phone to ring" option is in the first menu you'll see.
The interface to the contact list and dialer are as expected. The only thing that irked me was that when loading the SMS, history or voicemail screens, it has to wait while it loads the data from the Web. I understand why this is the case, but it seems to me as if this could be optimized (e.g. by showing you cached data while it loads more in the background).
Voice mails play fine, though they have a noticeable pause before they start. Nothing I can't live with. It doesn't show me the transcripts along-side the voicemail, but I already have that sent to my phone via SMS, so no worries there.
Overall, I like the integration, and it was well worth having to jailbreak my phone. Who knows, perhaps I'll even test out some of the other features like the "modem" and running various shells and servers. There's a lot Apple doesn't let you do with your iPhone that I really think is worth doing. Sad. Maybe I'll check out those Android phones anyway....