Beef bowls are to Japanese like fast-food hambugers are to Americans: cheap, fast, and kind of edible.
After the US beef ban since this mad cow business, chain beef bowl restaurants such as Yoshinoya haven't been able to do too much. There were some attempts at making chicken bowls first, and then the whole Avarian Influenza outbreak just freaked everyone out about anything with feathers, and that got cut out too. So, these guys had to restructure their menus around the joys of pork.
In my opinion, still tastes about the same.
May not be the same case for most Japanese, though. 70% have stopped going. These are dire times for fast food beef in Japan's drowning economy.
This probablly isn't effecting the US part of their stores that much, though.
It's open 24/7 in Japan, like most of the beef bowl (gyuudon) shops are. So, if it's 3am and you're drunk and want a little something to fill the stomach, head to Yoshinoya!