I've been using Google Voice on and off for about a week now, and wanted to share my first impressions.
Google Voice is still available only by invite, but I only had to wait about 3 days to get access. Signup was quick and easy, and doesn't cost any money. The hardest part was selecting a phone number -- you can choose one from most anywhere in the United States, but as far as I know you can't change it later, at least not easily. I finally got the Montana phone number I've always wanted!
Simultaneous multiple call forwarding: The feature I was most aware of beforehand is having your Google Voice number forward calls to as many other numbers as you want. This is actually better than I expected, because it calls the other numbers simultaneously, not in series, so there's very little delay to the caller compared with directly calling. Whichever phone you answer first and tell the robot lady "1", is the one that takes the call.
Android integration (not VoIP): I was most excited to use Google Voice as a VoIP client on my Android phone, but that is a feature I apparently imagined all by myself. The Android Google Voice client still uses cell phone minutes and goes through the cell network, but it routes the calls through Google's infrastructure and shows caller ID as your Google Voice number. The point of this is to save money on international calls because you are only making a domestic call to your Google Voice number as far as your cell phone provider is concerned, and then Google routes the international call and charges you a few cents a minute. (This is the only part of Google Voice that costs money at the moment, as far as I know.) That's a neat feature, but what I really wanted was a completely VoIP client that I can use on wifi or a 3G data network to make calls entirely outside the cell phone network. Any Google Voice developers reading: Here's my enthusiastic vote for that! I would use it all the time if it existed.
Google Contacts integration: Because I'm using an Android phone and its native contact management, my contacts are synced with my Google account and thus appear in Google Voice automatically. That is convenient. It means the GOOG has all my contact info. That's more or less the case with anyone routing phone calls for me anyway.
Free SMS: Google Voice does free SMS text messages, both sending and receiving, even if you have no cell phone. If you do have a cell phone configured with it, it also forwards incoming SMS to your phone. Very handy. Every cell phone SMS someone has sent me has made it, but for some reason I haven't been able to get Skype's SMS to go to Google Voice at all. That keeps me for now from having SkypeOut calls show a Google Voice number as the caller ID, unfortunately.
Free domestic long distance: Another nice feature I hadn't known about before is Google Voice's ability to initiate two outgoing calls, one to one of my configured phones, and the other to anyone else, and connect us. This makes it possible to use no long distance but call whoever you want. A feature I haven't tried yet is having others call the Google Voice number and conferencing them in.
Volume attenuation problem: The biggest problem I've noticed with Google Voice call quality so far is a serious attenuation of sound volume. When talking one-on-one with someone it's tolerable, and many people may not notice it. But when I called into a conference call with Google Voice people could barely hear me at all, when I talked loudly. I could hear them ok, but they couldn't hear me. I use that conference call facility all the time with regular phones, cell phones, and SkypeOut, and have no problems there. If Google could somehow boost the call volume to compensate for the extra connections, that might help. But for now I simply can't use it for conference calls, where I spend a fair amount of time every week.
Silent participants dropped: The second-biggest problem is that Google Voice apparently intentionally drops the call without warning whenever one side is silent for more than a few minutes. I haven't seen this documented anywhere, and Google doesn't tell you in any way, but it's pretty clear that's what's happening. As long as both sides are talking fairly frequently it's fine, but on conference calls I often have to stay muted for many minutes while others are talking, and then I'm unceremoniously dropped. So that's strike two against using Google Voice for conference calls right now.
Voice mail transcription: Voice mail left at Google Voice is available via the Android app, the web app, can be emailed to you, but most amazingly, if in English, is automatically transcribed and sent via SMS text message! It was hard to imagine this would work very well. I had Kiel test it for me. Compare his voice mail recording to the transcription:
A little cutie of the corner told me to leave you not Grandmama call messages and so the core in based cheese card test, kittens From, trucking, and you know from getting to fat until all the super caliper at July 6th and if that's not ballistic. I don't even know it's realistic. Because let's just face it. Corn is, where's that. See you.
Note that it displays the transcription lighter or darker depending on its level of confidence in each word. The transcription is not perfect, and Kiel deliberately chose words and phrases to be difficult to decode, but it's definitely close enough to give an idea of what the message is about without having to listen to the recording. Very useful if you're somewhere you can only peek at the phone for a few moments and can decide if the message warrants excusing yourself from the room or not.
Listen in: I haven't used this yet, but can imagine I would. You can let people start leaving voice mail but listen to it in real time and decide to take the call, just as with old answering machines.
Switch phones: I haven't used this yet either, but you can switch phones during a call from a land line to a cell phone or vice versa.
In summary, it is not the be-all and end-all of phone services that I may have hoped for, but it's a very useful addition to the world of phone services, is easy to get going with, and is mostly free of charge. It'll be fun to see where Google takes this.