I own one and I've bought for missionary friends oversees. I have teenage girls in my house (they don't have cell phones) and for almost NOTHING I've provided another line for them to use rather than them arguing about who gets the phone. Great deal!
I also have a LOT of friends who live overseas. Since this has UNLIMITED calling to US from ANYWHERE IN THE WORLD at no additional cost, it's perfect for them. They can plug it in anywhere they have high speed internet and call the US for free. And since it also comes with a US phone number (you choose the area of the country and the prefix), all their friends and relatives can call them the same as making any domestic call. Plus... in some countries, phone lines are monitored by the government... If two people have Magic Jacks overseas, they can call each other using these and it never touches the local phone system.
I would say for anyone who has friends or relatives overseas who need to call the US (or even businesses who do business with the US and want to call cheap) this is the best deal in the world!
It's also (as the consultant above said) an awesome way to carry your phone with you wherever you go. If you have this and even a little cell phone over the ear headset (with 2.5 mm to 3.5 mm adapter), you can turn anywhere you are with your laptop and broadband into a phone... or for that matter, if you are visiting a friend in another part of the country or world, you just plug this into their computer. Yes, cell phones are more convenient for that purpose, but just having the extra ability is a benefit, plus using a domestic cell phone overseas isn't exactly cheap.
Also, if people call you and leave messages, they (Magic Jack) send you an email with a sound file attached so you know that someone has called (and can listen immediately) without having to dial in or hook anything up.
Setup is simple, taking about 10 minutes total. The only negative (other than having to leave a computer on) that I've seen, is that if you use headphone and mic instead of plugging in a phone, it will tie up your sound card and you won't be able to play music and such. I don't believe this is the case with plugging in an actual phone in the RJ11 jack.
This is a good deal at full price. The sale price was just icing on the cake!
Joe