I don't mind gear sharing so much, but only my amp, it was cheap, it'a fairly powerful, i'm fairly confident nobody is going to break it and i've spilt a whole pint of coke over it before and it's fine. That's not to say people are careful with it. Are they f***! I usually end up letting everyone on the night use my amp because NOBODY where i live has a decent amp seriously! I suppose it's the price i pay for having the drummer (who helps organise the gigs) give me a lift to practice/the gig sometimes, but it still pisses me off when i see people turning it up way too loud or red lining it or just generally abusing it... And they do it surprisingly often! (i'm gonna wrap it up now cos i'm getting worked up!) Nobody plays my basses though, not for the actual show anyway. If they want to just noodle for a minute before/after the show, fine, but not during the show, no way. I'm letting my rhythm guitarist, who i do trust a fair bit, borrow my les paul on friday night for a gig, and i'm still wary about that!
As far as me borrowing kit though, i wouldn't want someone damaging my stuff, so i try and be as careful as i can be with other people's stuff.