I am an B820 owner. I traded a Janome Horizon toward it- never having owned a Bernina before. It did have a learning curve-- and because I did not buy it from a Bernina dedicated dealer, I found that I had to go take classes at a dedicated Bernina dealer shop to fully appreciate the machine.
A part of the issue I found in my own case was: both my purchase from dealer and myself at home, between the both of us, really did not have a proper table for a machine of that size, weight and power.
Originally I only had a six foot long cosco collapsible "picnic" type table. Well that is the sort of table my dealer hosts classes on as well-- The BSR couldn't get a steady read on the fabric with the machine movement from being on an unsteady surface, if I got too fast. I figured this out when I went to the Bernina machine shop (dedicated machine sales/not a quilt shop), and low and behold....once I experienced that machine on a rock solid surface....the angels sang. That was the secret handshake for being able to really let that machine rock to it's fullest.
I hate the wrap around slip on tables because for me, the entire work surface begins to get too high and it is not ergonomically correct. It makes long hours of sewing too painful. Having the flatbed ability now with a cabinet is another piece of heaven.
Tension has not been a problem in my experience-- the manual does recommend tension adjustments for certain stitches, and I just make sure it gets it's annual service. But other than those things, no real tension issues.
The threader does have a delay, and if you get too fast with it can be fussy. I have managed to get my pace and technique down to where I can get it threaded just about a hundred percent now, BUT needle is a factor. I like topstitch needles and they do have a bigger eye, which also helps prevent thread breakage issues regardless of machine. I have to concede: The Brother Company did invent the best needle threaders. My favorite is the kind with the lever up on the head of the machine, it's so funny, they put it on their cheapest machines...and it's the best built in threader ever...LOL...
I am tempted to trade for the 880, but in order to add embroidery cabability and all of those stitches, but after seeing that Bernina had to redesign parts for the 7 Series after their roll out...I wouldn't even consider it for a good 18 mos after the introduction, just to make sure they were doing good. But at the end of the day-- The 8 Series I think, really have a limited audience because of how they function, which is another reason why they are expensive. Bernina probably knows off the bat, they will not ever sell as many as the Quattros and Ellisimos...they seem to really be going for a very targeted audience.
If I don't trade up-- I will probably add a Viking. Their hoops are really aimed at quilters and they have some unique advantageous features that appeal to me.