The service looks a lot like Gmail for voicemail, although you don't have to be a Gmail user to use it. It lets you retrieve transcripts of your voicemail, archive and search all your sent and received SMS messages, and you can make calls directly from Google Voice's interface. The calls are free in the US, and cost a small fee internationally.
The automated transcripts are definitely one of the most interesting features; Google has invested a lot in voice-to-text and text-to-voice conversions lately, and this feature will probably be reliable enough for most users. If you like, you can set Google Voice to email or SMS you the transcriptions.
However, Google Voice can also be seen as an aggregator for all various forms of phone communication. It lets you pick one phone number, and reroute voicemail, SMS and phone calls from other to that number, or several numbers. Depending on the adoption rate of Google Voice, it could be a huge threat to a number of other popular services; VoIP clients such as Skype, as well as voicemail transcription services such as Spinvox, come to mind.
If you'd like an invite to Google Voice, try here; for a walkthrough, see the video below.