I have the Viking Sapphire 830 (previous model - only slightly different). I have had it for 1-1/2 years and LOVE it! The 855 will have more stitches and maybe the thread cutter, but not positive about that. Hopefully someone else can confirm.
Having the extra space is great for larger quilts. I wasn't having much luck with the "R" foot for FMQ, and took it in for a lesson. The store owner gave me a spring foot that works much better for me.
It's different in that the settings are automatic based on the stitches you choose - including presser foot pressure. You can alter them, just wanted to mention it. There are actually 3-4 needle positions and if you don't have it all the way up, you can't move the top thread. There is no lever to raise the foot. It raises with the needle down when you stop.
I have been very happy with it and would certainly buy it again if the need arose.