I just recently bought the Magic Jack. Here are my pros and cons:

Pros:
1. Call quality is good
2. Phone bill is only $20 a year after you buy it for $35-40
3. plug and play super easy installation. No disk, just plug it into USB and it installs
4. PC keeps a log of all calls ingoing and outgoing
5. you can get e-mail alerts to advise you that a voicemail was left
6. you can dial from your PC and talk on a headset which is kind of cool
7. Free 411

Cons:
1. It has to be plugged into a computer that is on. Your phone can't call unless you computer is on. That stinks. But your caller will still be able to leave a voicemail.
2. It is so simplistic, there could be a lot of other features on the PC interface but there isn't much.
3. Caller ID displays "unknown caller". Some people may like that, but not having the option to choose the name displayed is a con to me.
4. Separate registration for 911. Plus, regarding 911, see con #1 above. If you are having a heart attack you may have to wait for your computer to boot up before you can even call for an ambulance.
5. They push their own advertising on the computer interface, advertising vanity numbers, international, and buy additional years of service. Asking on their website is one thing, but they don't need this type of advertising on the dialpad - sheesh.

Overall, I'd say it's good as a second line. For $20 a year, that's a heck of a lot cheaper than other options.