I will just show you some sites.
http://www.sewalot.com/singer_201k_sewalot.htm
http://blog.sew-classic.com/2008/11/...ne-review.aspx
http://www.ismacs.net/singer_sewing_...-machines.html
and I could go on and on. My personal reasons: the large harp for quilting. The trouble-free potted motor. The tension dial on the front(one thing I hate about class 15s). Negatives: don't drop feed dogs, I have to cover mine to FMQ.
however, I do have a pink Atlas, a pink Monarch, and a blue Remington for when I want color(not to mention the repaints). The pink Atlas is my favorite- it has the tension dials in front, and a handy knob to drop the feed dogs. The others are class-15 clones.
Most of the japanese mid-century machines drop feed dogs and have front tension dials. That must by why I have 9 of them- it is my favorite configuration.