Well, things are looking up: You can now get Google Voice on your iPhone more or less the way Google intended it. Don't believe me? Check out the video.
No, there's no Google Voice app for the iPhone yet. Maybe there never will be. But now you can use the Google Voice webapp instead. I've been making and receiving calls using Google Voice on the iPhone for several days now, and it's a joy to use. The webapp delivers a dialer for placing calls, a contact list, inbox for your SMS messages, voicemail, and more.
You don't have to enter any confusing prefixes or surf the Web to place a call; it's just like using your normal iPhone dialer, for the most part. Callers see your Google Voice number, not your iPhone number. And when someone calls your Google Voice number, your phone rings, whether the webapp is running or not.
Finally, Google Voice is a first class citizen on the iPhone, and there's no reason left not to try it out. You can read more at the Google Voice Blog.
And if you're still on the fence, read 8 things you should know about Google Voice.