I’m talking about Narita. My girlfriend and I are on our way home to Taiwan from the states, but we’re stopping in Tokyo for three day to stay with a friend. We don’t want to lug four months of bags into town and back out, so we’re thinking about leaving some of them in storage at the airport. I’ve looked, and it seems to be possible.
My question is, has anyone done this before? Is it a hassle? Any advice would be welcome.
I’ve never personally used that service, but, given the pretty outstanding quality of customer service at Narita, I’d say the process is probably a cinch. I’ve consistently found Narita to be one of the easiest to deal with airports I’ve been through, although it can be easy to get lost in some wings 
I’ve used these sort of services before, but always in sending my luggage from Tokyo ahead of time. The longest I’ve left it there was longer than you’ll be there, so that at least is not a concern. It’s possible that I’ve used Greenport from your link before because the name seems vaguely familiar, but I can’t be sure. I can tell you that it’s always been trivially easy for me when I’ve used them, but not much more than that.
I haven’t used that service in Tokyo, but I’ve used a similar on in Amsterdam, London and Zurich. I’ve never had any problems with the storage, and drop off/pick up were a snap. I’d do it again in a heartbeat, to avoid hauling the extra luggage around for 3 days.
Something else to look into. In Zurich, there were self-serve lockers right outside the manned room where you could store stuff. If we’d had the coins when we landed, it would have been much cheaper for us to rent a locker for 3 days, then to use the storage service. That’s mostly because of the large number of bags we were storing. So keep an eye out and watch for the lockers.
I’ve used the storage service once or twice, but only for hours at a time, not days.
I don’t recall seeing “self serve” lockers at Narita