I joined Trusted Housesitters about 4 months ago (because we have a dog) but I have been too nervous to use it.
My son pointed out that probably 99% of the time there would be no issues but what if 1% of the time the person house sitting decided to change your locks and squat in your house. It would not, apparently, be a criminal matter but a civil one, and it might take months of expensive county court procedures to regain your home - and in what state!
Of course, that is the alarmist perspective, and some of my choir friends have used this organisation without any problems whatsoever, but it scared me.