I have a featherweight because of several reasons:
1. I learned to sew on my mother's (which my sister now has) and the nostalgia is a strong pull.
2. It has a beautiful straight stitch, but so do all of the vintage straight stitch only machines.
3. It's very portable. Weighing in at 11.2 pounds, and with it's compact size, it's a breeze to take it to sewing groups or classes.
4. I simply like the looks of the old black Singers. The white and tan versions don't do much for me, altho I wouldn't turn one down if it was given to me, or I found one at an excellent price!
I don't know that there's anything terribly special about having one. Any good machine will do what the Featherweight does....sew a straight seam. Modern machines will do a lot more than that, without having to use attachments to do it with. Definitely a "luxury" item.
What to look for when purchasing? Depends on what you decide you want. Since I couldn't have my mother's machine, I decided that I wanted one that was made in 1956, like me! ;) Remember that the older Feathers had prettier faceplates with scrolls, while the "newer" models had straight line faceplates.
Otherwise look for the things that you'd look for in any used machine. Does it work? What comes with it? Look at the wiring, is it intact or will it need rewired? Are cosmetics important to you? You'll get a cheaper price (or should) on a Feather that has rubbed off decals, scratches, dings, ratty case, no attachments, no manual, etc. For one that "has it all....expect to pay more. Keep in mind that the attachments are "a dime a dozen" on ebay, with a few exceptions. (I just paid $25 on ebay this morning for a large assortment of attachments, along with a buttonholer and a zigzagger...mind you, I already have 99% of these, but can always use more (a set for each machine in my collection!!) AND, with all of the attachments that I already own for vintage machines, I did NOT have ONE ZIPPER FOOT!! :( )
It all boils down to the degree of "want"...you don't really NEED a Featherweight...ya gotta want one! ;)