I think that when someone has issues, they are more likely to write a review than someone who's happily sewing along.
I have a Viking Sapphire 830. It was a demo model that I bought last year. I have not had any issues with it. It just had it's first check up and everything is well. It seems to prefer certain brands of thread, and that's fine with me. As long as she's happy! LOL I think the main problem people have with the Vikings is that it's auto everything and takes some getting used to. From some of the things I've read, it could be user error in many of the cases. The presser foot doesn't just go up and down. There are a few levels of height. If it's not all the way up, the thread will not move! Once you get used to that, it's all good...and I do still occasionally reach for the presser foot lever! :) It's great to have the larger throat. I have no regrets!