Disklavier

Am I happy with upgrade?  Yes very much.
Is it worth the cost?

Well given the overall cost of the Grand Piano. It is a small percentage.  But it is still expensive.

Was it worth the cost and time of transition?   It was the easiest upgrade I have ever made.  Can be done in 15 minutes.
It comes with a new bracket that fits perfectly. You simply pull out the old connections from Mark III and plug into new 850 connections.  Then sort of stuff the wires under the piano.
I already had a USB floppy A Drive and it easily plugged into the Disklavier.  You can load disks through the Floppy A drive.  
Also if you have a collection of midi songs;  you can load them into the 850 from your computer very easily. Once you have set up the INTERNET connection for the YAMAHA radio,
You will see your 850 on your computer's shared network devices as dkvxxxxx.  As you click down you will find a folder "From to PC" and it is there that you can copy your Midi files from your computer.

With  the 850 remote you can move these copied files to other locations on the 850.  But the folder "From to PC" is fine with me. The "From to PC" is just another source (Like CD) to the 850.

I think it works great and is very versatile.