The 400 and 500 series Slant-o-matics have a pretty good following outside of the vintage collecting crowd too. They go for higher prices than average machines. I've seen the prices creep up on them over the two years I've been interested in old sewing machines. The 401As are very strong, smooth running machines that are respected outside vintage circles and the 500 series Rocketeers are popular for their looks. I'm guessing they run about the same as the 400 series machines as well.
I'm not quite sure how to word this. They were made right at pretty much Singer's peak, at least for domestic machines. I'm not sure of the industrial side of things. Shortly after Singer made these machines they started struggling financially and the quality of their machines suffered as a result.
I think these machines were pretty close to being at the peak of mechanical sewing machine technology for any brand. The only other significant mechanical advances I can think of are the Touch & Sew's autofilling bobbins that didn't really catch on and the addition of a second cam follower to move the feed dogs back & forth making stretch stitch and other decorative stitches possible. The downside is plastic gears and camstacks were introduced at about the same time.
I could go on.
Rodney