Hi! I've had my DX-7 for about six weeks now. I love it. I have used it for piecing, FMQ, applying binding and decorative stitching on crazy quilt blocks. This is my first computerized machine.
I love the setting for a scant 1/4 in. seam. The height-adjustable quilting foot is great. I also appreciate the stitch-in-the-ditch foot. The machine is very quiet, and the needle threader actually works. I was never able to get the one on my Juki TL2010Q to work ...
I have used different threads, cotton, polyester, embroidery, metallic, with no problem.
I just wish they had made the harp larger. There is 8 in. to the right of the needle, but the harp is not very high. Quilting a large project might be a little difficult, but could be managed I suppose. I will use the TL2010Q for large projects.
I'm not sure what the differences are between the F400 and the DX-7. You may already have many of the features of the newer machine. The newer machine may have additional feet and more decorative stitches. I'm not sure.
Do let us know if you decide to go with the DX-7.