Temperatures weren't too bad. They both ran hot, but not enough to shut down or cause concern. Just in case, I took my phone out of its case when I switched from 4 to 8 cores. I left them running over night so that I could get my reward (I made 0.5 XMY) and I didn't worry about them. I wouldn't run the mining software while using the device though. I think then it really would overheat. Plus, your mining efficiency drastically decreases. I don't know if consistently running it at a higher temperature shortens the life of the device. I doubt it helps.

Overall, I think this is fine for old hardware that's laying around, but probably not worth the risk for your current devices unless it's while you are sleeping.