Harrkey, I'm not uncomfortable using these old beasts as handwarmers while hunting or camping. I'd be uneasy using one in a tent, because even though they're "catalytic" heaters they need to be ignited with a flame and then "burn off" for a while. Never had an air-tight tent, so I doubt they'd generate dangerous levels of CO. However, my experience putting them in the foot of a sleeping bag (years ago) is that they quickly fail and cool off (perhaps because they do need SOME air).
Also note, I've had several of them leak or leach Zippo fuel: anyone who's had an over-filled or leaking fuel lighter in a pocket can remember the burn!

Have had no experience with the solid fuel types.