Google Voice is a great service, I use it all of the time. It syncs well with my smartphone (Android, of course).
Not only is it good for international calls (extremely cheap compared to carrier rates, and I make far too many calls to Canada), it's good for people you don't want to give your real number to.
If you are using your cell phone, you have to be in 3G coverage, basically. There are issues with call quality, but I've never had it be so bad that it's unusable. I give it out on a regular basis to people that I don't want to give my phone number to.
Visual voicemail is also wonderful. I like just reading my messages. It's not the best voice transcription, but it usually works well enough to get the point across.
Also, making calls from your computer is always nice, if a novelty at some points. Also, things like ringing to certain phones in certain hours, so that it calls your home and cell, having people say their name and then accepting the call based on that, and a ton of other stuff do make it useful.
I'd say use it. It's free, it gives you a number in the area code you want, and it works.