I've been to a good percentage of the Hard-Offs in Tokyo, and they are more often than not disappointments. If you're looking for PS2, PS3, or any flavor of DS hard/software, you're gonna love it. If you have a taste for anything else you will be sorely disappointed. If you do find some PS2 games that you like, expect eBay prices. I saw Dodonpachi last Monday for Y5,000. I have a feeling that all of the stores in Tokyo are picked over by every foreigner / English teacher that comes here. When you get to one, you'll see what I mean.
Shopping is Akihabara for retro games is a joke now. May as well stay at home and see what you can find online. Prices started getting ridiculous in about February 2012. I really only go there now for PCBs, but STV has pretty much ruined that.
If you can make it out to the countryside, that's where the deals are to be had. Boxed Turboduos for Y10,000. CIB famicom games for Y500, I got an MSX computer with ~20 games last week for Y5,000. Boxed famicom with all papers for Y2,000. Boxed super famicoms for Y4,000. Amazing finds at nearly every store. I wanted to get a boxed PC-FX that I found for Y2,000 but I ran out of money.
Hardoff has a sister-store out in Saitama called Hobby-Off, and the deals are amazing. I make it out there twice a month, and come back with a trunk-load of retro treasures. There's also another store called Toy Planet that only sells used toys and games, and the prices will leave your jaw on the floor. Its a giant warehouse, and looks like costco for old games and toys. The only problem is that you'll need to rent a car, or make friends with someone that does.
Toy Planet:
http://www.toyplanet.jp/
oh, and then there's this place:
http://www.beep-shop.com/
I wont even talk about that since it's waaaayy in the countryside, and you'd never go back home if you found it.
Have fun while you're here!