Regardless of your VoIP service, you're being charged by the month. MJ charges by the year, saving money by a factor of 10-12. If the quality in VoIP isn't ten times as good, then unless you need the higher quality, MJ is the money-saver.
I just use mine for a voice mail number. It's cheaper than voice mail from a landline, and it emails me when someone leaves a message, which I can play on my computer without even having the MJ box in the USB port. I can make free LD calls from any PC running Win-Don'ts as a fallback to cellular.
According to posts here, you can run MJ on Linux through VM. I haven't tried and probably won't.
It's a decent enough service, especially if you don't use it as a primary phone. What do you want for twenty bucks a year, AT&T landline quality? Most cellphone plans offer free LD. VoIP and MJ both are pretty much obsolete anyway. Once everyone realizes "no contract" is their biggest selling point, all phone plans will be as interchangeable as soft drinks. Go with what works for you, and if other people don't choose as wisely as you think they should, taking more than one post to argue with them just adds grief to the mix.