I just had my new QE 4, bought in November replaced with another one. I couldn't get it to FMQ at all - bobbin breaking and skipping stitches on the back (could FMQ 'perfectly' (lol) on my Janome).
Whilst it was in the shop I did a lot of net surfing and was surprised at how common this problem is. A lady in my group cannot FMQ on hers at all - same problem I had.
The new one is great! I like the the Sensormatic so much I haven't even tried the spring/darning foot.