The evolution of this kind of lens is primarily from the birding side. Birders use spotting scopes and then started using various adapters to fit cameras and then DSLRs started getting attached. The quality level obviously cannot be expected to match that from a DSLR and a long tele lens.

So I am sure a few people may be buying it. I am not aware of anyone who is using this lens. So you need to find out actual user who is using it. It becomes difficult to sell it if you don't like it. So you need to do a bit of research on this.