Medicine ball, yep been there done that. I don't think it hurts anything as long as they are dropping not throwing it.
We do the crunches with Thai pad beatings more often though and I think they are better. Let me explain if you haven't done these. One partner lies on the ground doing regular crunches with his hands behind the head or atleast out of the way. The other Partner kneels next to you and rythmically pounds your abs with a Thai Pad. The rythm is once per crunch on the way up. This teaches body conditioning ab work and timing all at once. Try it you'll hate it, then love it.
I've done something like that. Except what I did was I hung a sandbag, pushed it away from me, let it come back towards me and hit my abs. That may seem a bit extreme, but it worked great.
Medicine ball, with a incline bench with someone throwing me a heavy medicine ball above my head i catch lean back do a sit up with it (on the inclinve bench) 20 times, then have someone throw to my side i catch lean back and throw back, each side 10 times, (do this drill 4 time) rotating with your partner.
oooooo, getting someone else in one the action! yeaoch!
several weeks ago i did 2 sets of weighted crunches on the decline (slowly, good form i think)
and 1 set of leg raises
burned for 3 days
but just doing crunches or situps on the ground is fine and nothing much for me....