I never used to mind lending out gear but I've definitely got stricter in recent years. If someone is a bit up themselves I'll now say no. Ironically the biggest name to use my gear was Norman Watt-Roy who was really grateful and gracious - unlike some of the posers who've happily rested pints on the head.
Not an amp but my worst experience was in Camden a couple of years back touring with an American band and the support band's bass packed up. I lent the bass player my bass to get them out of a hole and afterwards he was really rude "I'm a guitarist really, you see", "it sounds horrible with that foam under the bridge - I wanted to rip it out", "don't like the flatwound strings" and "why do you have those covers on it?". Funny thing but my gear lasted the tour, his didn't last the set. Go figure!?! If I'd have known he was going to be like that I'd have let him sing his bass parts.