I cut up 2 old dacron genoas to make my 320sq'/30sq m sail. I think it worked out pretty good, but I've only just finished rigging it on the mast and hopefully I'll take it for a sail this weekend. I followed Arne's step-by-step guide and that was very helpful. The used dacron has more stretch than new dacron, but probably much less so than the odyssey. Mine has barrell camber, I think the dacron might have set better with shelf camber. Mine is a little wrinkly, but I'm still getting to know the set and adjusting my HK parrels, and turns out my wood battens are too flexible anyway.
I can't speak about how strong the used dacron is, but I think it will be fine for the sailing I'm going to do. I'm happy with the result, it was cheap, and now I have a lot more experience with constructing these sails. I think that's the important part. I learned so much by doing it that I never imagined while reading and re-reading PJR and Arne's CPJR.
As far as working with the cloth itself, I used basting tape to put the panels together, I think Arne recommends staples. I found getting the panel edges to match was sometimes tricky, if you stretch one more than the other when stick-fitting them together you'll get a mis-match by the end of the panel. The other thing to deal with is the old seams and reinforcements. Some of my panels have seams and bias running all over the place; the pattern looks interesting and no problems so far. The reinforcements I just removed.
My mantra when I was working on them was "how bad do they have to be before they won't work at all?" but I think they worked out better than that; we'll see this weekend.