Much works fine on the MFD12, but there are a lot of bugs too, and all appearances are that Furuno doesn't plan to fix them. Their last update to the firmware was over 2 years ago, and all their attention appears to be on the TZ models now which get updated regularly. I've been pretty disappointed with my NN3D system and am switching to Simrad on my new boat.
Here's a link to our blog talking about it. Adventures of Tanglewood: Electronic Plotters
There are a few other posts later on which provide further info.
This is all just one person's experience, so judge for yourself.
Also, you should go to the Furunousa.com web site and go look at the support forum. Read through the section on Navnet 3D, especially the parts asking when the next fw release is coming to fix the bugs. There are some very unhappy customers, many much more so than me.