Value I don't know, but I have been on the look for a vertical feed. There are some info on the Victorian sweatshop forum, a few videos on youtube and there is a couple of blog posts on the web. I guess there are a few threads on this forum too, but at the moment the search function doesn't give anything. The manual for the early version of this machine is on
ISMACS.
I took an interest in this model because of the upper feed function, and I know it often needs some extra attension on the parts behind the face plate. From a couple of people ( probalby still active on this forum) I have picked up a bit of info regarding the roller action and movements of the parts connected to the presser foot movents. It seems to be a good thing to know in advance when taking one of these back in to use after years of storage. The rollers are suppose to turn, and all parts require a good application of oil and maby cleaning.
The blog post on this model are often from quilters who report the upper feed is ideal for quilting layers of material and cotton wadding. The presser foot lift and harp space is rather generous. I think someone managed to quilt with top stitch thread too and it gave nice stitches. The trick was to wind a few bobbins in advance since the long bobbins doesn't hold as much thread as round bobbin models.
There aren't many vintage upper feed models available, and no all metal machines except this model. Most machines need an additional walking foot, and in comparsion a Davis Vertical feed is more capable when it comes to feed and layers of material. A domestic sized machine is often preferable for home use, since industrial walking foot macines tend to be much bigger and heavier.
Some have found the Davis vertical feed too noisy, but those who have ended up using it have managed to quiet it down to acceptable levels. I guess it's all about oiling (teflon oil?) and the right kind of cushioning felt pads under the feet.