When I try another guitar I can't play it as well as my own simply because the context in which I am likely to be using someonelse's guitar is a context in which I would feel less comfortable. I don't feel comfortable playing in music stores (when I'm not buying anything) cause I don't like ramming my decidedly minority style music on innocent shoppers! Another thing; tone's never so good if you underpower your amp as a self-conscious guitarist like myself might do. I bought a new guitar recently (secondhand Ibanez Rg570) and it took me very little time to get used to it when I got it home.
As for friends guitars they can be harder to play than my number one axe (afore mentioned) cause 12strings/acoustic/semiacoustic/wideneckjointed models are harder to play than a nice straight-forward lecky with a low action.