I've had an MJ+ installed for the past year. It's crashed maybe three times in that period. I don't use it for my primary phone and have incoming calls forwarded to my Bell line.
I also wouldn't consider it as "reliable" for 911 service with an 88 year old senior living in the house.
For free North American long distance and a few calls per month to Europe, however, I find it pretty adequate. Sometimes it will take a couple of tries to connect for no good reason and sometimes I've received complaints about call quality. A few times calls have been dropped, again for no good reason.
However, 99% of the time, it's good enough to make free long distance calls without any problems and has saved me a bundle on Bell long distance this year. Furthermore, I've just installed the MJ app on my Toshiba Android tablet and even though I've tied it to my account it's capable of making outgoing calls free of charge even without a MJ account so I have a handy way of placing long distance calls wherever I can find a wifi hotspot.
In a nutshell, I wouldn't consider it reliable enough to be a landline replacement but then again I don't consider any VOIP solution to be a landline replacement either.
If you want to use it to scrap your LD plan and save a bundle however, I highly recommend it. Just as long as your expectations are reasonable.