I loaned my infant seat to a close friend. She, of coarse, used it right and I got it back in great condition, but I knew that she would take care of it. We are pretty close since I was there for her whole labor and birth and babysat before "grandma" did. I figured it would save her some money towards a convertible seat and it would just be sitting in my attic anyway.
I would probably loan my Evenflo Chase to someone who needed it, but it would have to be a special person indeed to get my Britax, Recaro, or Radian, especailly for more than a week or so ;-)
In general when I loan anything out I ask myself how I would feel if I NEVER got it back or get it back in unuseable condition. If I would be mad or not want to be their friend anymore, I just won't loan it out. For instance, I loaned my $12 umbrella stroller to our neighbor's 20 yr old daughter when she was babysitting and begged to borrow a stroller. I never saw it again. Sure, I sort of wanted it back, but it's not a big deal- and I got a Zooper Salsa after that since I never really liked it anyway. LOL. Here DH just htought I was being forgiveing...