Despite the look of Viber, this app really does nothing special unless you have friends using the app as well. I think my main concern with this app is that the offering of free calls and texts is already a part of most smartphone plans. I can't speak for my friends across the pond, but here in the US, all four major cell companies offer free mobile to mobile calls and texting, either to others on the same network or, for me, any mobile device.
Now, if you wanted to be technical about it, I'm paying for this free service of calls and texts and that's where I think Viber fails here. It would be one thing if Viber Media, Inc offered this as a desktop or even product, like the Magic Jack, and then offered the Android app as something supplemental, but again, the only way I see this working is if you're not on a major cell phone network. Cricket users, for instance, might find this to be a worthy app. I'm not familiar with the changes that Cricket has gone through, but I do remember that they did only offer local service, such as calls within the city or state.
I could also see you using this if you travel a lot, especially out of the country.
However, for anyone who wants to use this app on a regular basis, I'd stay away from it. If you're paying your $70-90/monthly cell phone bill, this is just a redundant app that will look pretty, but will still use those minutes you're paying for. Again, this is only viable if you get friends and family to sign up for it and only if they aren't getting free mobile to mobile or unlimited texting already.