Hi, this is a very interesting thread. I recently purchased a Viking 835 and am very happy with it. I have always owned Pfaffs so I was a little hesitant to make a change. My dealer offered a fantastic price ($750) so I thought I'd try it. She told me that some (not all) of the Vikings in the 830, 850 and 870 models did have tension problems when the line was first introduced, but that Viking corrected them and then changed the model numbers to 835, 855 and 875. She also said that there was an older style 1/4 inch foot that would also cause some problems but the new foot is fantastic. I don't know if that is a story, but it's what she told me :).
Anyway, I am very impressed with my Viking 835. It lays down a perfect stitch and I LOVE the buttonholes. They turn out simply perfect every time. The FMQ is a breeze. The pressure foot seems to float on the fabric like magic. The extra lighting and 10" throat really make the machine a keeper too.
Mary