Google Voice lets you create a generic phone number that can be used to make and receive voice calls, voice mails, or text messages from any number of different mobile devices. It's great for people who want a consistent contact number across multiple phones and tablets or for business owners in need of another line.
To get started, you must first enable the Google Voice service on your Google account at google.com/voice. When doing so, you can choose to use your mobile number as your Google Voice number or create a new number altogether. Either way, you'll be able to use this number to make outbound calls from any of your Google Voice-enabled mobile devices. Meanwhile, incoming calls to your Google Voice number will be routed through the app.
Once you've gotten yourself set up through the Web, go ahead and download Google Voice on your Android device, and give it your Google credentials. From there, the app lets you select its method for making phone calls. For instance, you can set Google Voice as the default service for outgoing phone calls. As for me, I like the app to give me a choice (Google Voice or carrier voice plan) before I make any calls.
While the Google Voice service as a whole is great, the app's interface is another story. Primarily, it's made up of a single screen with a list of your messages... and that's it. There's no list of recent calls made or received, and there's no way to quickly make calls to your favorite contacts from the app interface. For some odd reason, these basic features are included in Google Voice for iPhone, but not in the Android iteration of the app.
One thing Android tablet users get, though, is Click-to-call, which lets you use your tablet to "place" Google Voice calls. With Click-to-call, Google Voice will call you at a number of your choice, then call one of your contacts, and connect the two of you. You probably won't use the feature very often, but I can certainly see it coming in handy.
Overall, I think Google Voice is a fantastic service that's worth signing up for. But until the app's interface gets a major update, I'll have to hold back on giving it any outstanding marks.