I was just down in Palm Springs and the place we were at had a magic jack. I was reasonably impressed. As Murray said the call quality will vary based on internet. This will happen with Skype, vonage, viber, or Magic Jack. I have used them all and had decent success.
As Murray said the 911 part is an issue. Personally I like using my cell phone and just putting one of the apps mentioned above on it and then using wi-fi as data roaming is very pricey.