Depends on the thing, and depends on the person.
I have a couple "spare" machines; vintage steel beasties I've picked up for a song at thrift shops; those I will (and have) lend out, no problem. If it's a $15-$20 investment and someone I reasonably trust...I can live with the loss if it happens. I also have a few spare/duplicate tools I'm OK with lending out, to the right person. Basically I don't lend out anything that I would be really upset to lose.
My niece and nephew have been allowed to run amok in my sewing room a few times; they're reasonably careful and I'm pretty confident they won't actually break anything; at least not deliberately. I usually have to adjust tension after they leave and replace a bent needle or two but so far all the machines have survived their attention and nobody has sewn their flesh or lopped off a finger.
To me, the risk of them damaging something is more than offset by the thought that I'm letting them learn the joy of sewing. My niece in particular LOVES my 6600; whenever she uses the scissor cut button she gives a happy sigh. She told me once that sound is "the best sound in the whole world".
I agree! I can afford to repair or even replace the machine if she somehow really mangled it, so I enjoy the luxury of being able to just turn her loose to experiment and play, like I did with my gran's machine when I was her age. It reminds me that *I* should play more often! And so far she hasn't hurt any thing besides needles. They're good kids.
I've borrowed things during classes a couple times, but just to try them out. I'm very careful to use things briefly and return them promptly and with thanks. I don't like borrowing things in general but sometimes I like to try something out before I buy it.