For the last year or so I've been doing a test with So Fine (50) as a bobbin weight and aurifil for the top thread on my fussy Bernina (aka Cranky Bernie). It's been doing very well with the So Fine and I'm happy with the results and ease of use. I've had to adjust a tension here or there but I typically piece with a 10 needle and a small stitch and like it very well for that. I also quilt with a 14 needle and a reasonable quilting stitch -- it likes that too.
I recently won a local auction for a ton of So Fine spools and will consider some trials with it as a top thread too, but not so sure on how the cones will work with my fussy machine.
The modern Bernina 820 "sewing computer" belonged to a friend who wanted to do much more fancy stuff with threads and this machine was just not working out for her. I had it and used it will with my more basic threads and such, but it still was fussy with thread and error codes and finally broke down completely. Even after getting it cleaned, serviced and the thread sensor replaced, it still would code on me randomly every 5-15 minutes of sewing time. Usually there wasn't anything actually wrong, I would tug down on the thread a bit and away we'd go until the next random 5-15 minutes.
Anyway, I was watching the inauguration and I had the thought that "I wish my Cranky Bernie could be made happy with a hat and a pair of mittens". And it came to me -- a hat! Also known as a thread tension disk, one of those saucer sort of things. I didn't have any in my case of accessories but I threaded the spool up through the bobbin (the bottom is solid, thread comes in from the middle) and out the top and it provides just the added amount of tension that I can now sew an entire bobbin with no error codes and the tension is better too!
edit/ps: Think by spool shape that is Connecting Threads and not Aurifil currently loaded on top.