I signed up for Google Voice a few days ago. Based on what I've seen so far, it has potential. You get a free phone number and a few services that you might find handy. Call routing to other phones based on incoming number ID, spam filtering on unwanted numbers, and speech to text that sends voice mails to email. I assume they're not using CSRs in call centers to provide the transcription. There's a free SMS feature but I haven't tried it yet. Pretty nice.
Even if you only use the free phone number, it's nice to have. You have complete phone number portability, since you can forward all calls to whichever phone you'd like. Don't care for your current mobile provider? Get a new one, and forward your calls there. Customers' switching costs are essentially zero now, as long as they don't abandon their current provider before the end of their contract.
Google has real plans for voice. This is another market they're going to fight over with Microsoft.