I had a friend that had one of those drill press attachments that he got at a yard sale.
I suppose that it worked OK but, the setup was so time consuming that you almost lost interest in doing the job. If you only had four or so mortises to do it was almost easier and faster to cut them by hand.
If you are going to do a lot of mortises, say 10 or more, I'm sure that it would be worth the setup time.
Also, make sure it will fit your drill press. My friends drill press was too small for the attachment and we had to put it on my drill press to try it out! Neither of us was impressed.
Rog