I made a few kallitypes on Clearprint Design Vellum. Had a nice black and you could probably do some interesting things with the translucent quality of the vellum.
However, it wrinkled a lot when I coated it, required more careful handling than heavy watercolor paper and the finished print was very wrinkled after drying. If I had a mounting press to flatten it out I might play with it again, but I could not get the wrinkles out to my satisfaction.
Like Ann said, some get really fantastic results. A pad of the paper is inexpensive, so it is easy to give it a try.