I didn't have one last summer in Europe and there were at least several times where I could've gotten a few euro discount if I had had it. So, I guess it depends on how much you can get it for. I would say if it's $20 US or less, it would probably at least pay for itself.
Flight to Tokyo: 92,000 yen
Metro to Ginza shopping district: 160 yen
Watching middle-aged Japanese woman dance to U2: priceless