I also use Skype also but recently found several apps on android phones where you can get a second number on your cell for around $20 a month. This way, you can separate business and personal calls easily. evoice is one of them, I have fumber and synclio isn't bad either.
Most of the people I know who use virtual phone numbers have them with Skype as it's relatively inexpensive to setup and the call quality is usually pretty good. I have no experience with google voice since I'm in Canada and I believe it's a U.S.-only thing at the moment.
For video calls, I also alternate between Skype and Google Hangouts depending on the client's preference. Although there are services that provide more stable and higher quality service (especially when connecting through long distances), free is a pretty hard value to pass up.
I've heard good things about Google Voice, but read that they will be ending service? I'm considering using Grasshopper as an alternative, although it's not exactly internet-phone service (I use Skype for that), because it can forward calls on a unique business number to my current personal cell phone and show me that it's someone who called my business number, and I can also call OUT on my personal cell phone with that business number so my personal # doesn't show up. Does anyone else use Grasshopper?