We got the Magic Jack about 2 months ago. We don't have a land-line, and use our cells. Our cells have poor coverage inside our apartment, so I was looking for something cheap to supplement our service when at home.
It worked great initially, and I was really impressed. However, as I made more calls I found out it sporadically doesn't work. What I mean by this is occasionally I will make a call and instead of the call going through the line goes to a dial tone or a busy signal. Also, once in a while the other caller complains that they can't hear me well. And, one caller said every time they call me they just get a fax machine sound.
So, it's certainly not as great as I initially thought it was. However, it serves our purpose in that for the most part it works and allows me to walk around the apt while having a conversation. If it doesn't work for some reason, I can always call back via my cell from my one spot in my apt where I have coverage or step outside for a moment.
It's worth it to me, but only because it is so cheap, and overall offers me more service in my house than I had before. IMO, you get what you pay for.
BTW, voicemail works perfectly fine so far.