I don't have an invite...yet. But I am sharing an account with a friend, and I have to say that, despite some privacy concerns, I expect to be using Google Voice in the future. The main reason is that I don't have a cell phone. Yes, that's right, I don't have a cell phone (not yet, anyways). Last year, I purchased the SkypeOut unlimited calling plan at $8 per 3 months while I was at Hopkins so I could call home and friends. However, with Google Voice I can now make calls for free within the continental United States.
But wait...Google Voice connects two phone endpoints. I have a Bluetooth headset that I can use with my computer. So I can use Skype for free PC to PC calling and Google Voice for free phone to phone calling (dorms have phones that are free for campus and incoming calls). But what if I want PC to phone or phone to PC, if only so I can use my headset? As it turns out, Google Voice can interoperate with the Gizmo SIP protocol. A Gizmo5 account can be used as a Google Voice phone endpoint, and has a Skype-like program that runs on Windows (and other OSes too).
Gizmo5, however, doesn't have a Windows Mobile Pocket PC client. However, it is SIP-based, so any SIP client can interoperate with Gizmo5 accounts. I found that the only SIP client to work decently on my Windows Mobile 6.1-flashed Dell Axim x51v was SJPhone. (If you use it, there is a caveat. The PPC 2003SE version doesn't hog the CPU like the unstable/beta for WM5 does, but the little notification message for incoming calls doesn't have buttons for Accept or Drop Call like the WM5 beta does. I went with the 2003 version, and mapped those two functions to hardware keys, since the only other way to answer a call is to enable automatic answering. It does, however, mean that those keys don't launch their usually mapped applications while SJPhone is running.) To dial using Google Voice on my PDA, I can either use Google's mobile web interface or the 3rd-party iDialer application (check out the companion iContact too). The latter is a little nicer to deal with, but if Internet Explorer is not the default browser on the PDA, it doesn't work. It's also a little buggier than the web interface sometimes.
But otherwise, I seem to have found a decent and free Skype-replacement for calling regular phones. The only other missing feature now I think is the lack of support for my special headset features (answer call, hang up, redial) that Skype supports.