Beware of a hangover
Happoshu, for all intents and purposes, is cheaper beer; or more specifically, "beer flavored malt beverage". Taxes on beer are somewhat high in Japan, and many are not willing to pay around 3$ U.S for a pint of beer even in a can. The solution is Happoshu, imitation beers that have as many varieties and options as regular beers. One may not even realize that they aren't drinking 100% real beer, and to be honest, many just stop caring altogether.
One thing to keep in mind however is that the cheap "all you can drink" offers at restaurants will generally use this happoshu imitation beer. The brand matters to a certain extent, but as a general rule, happoshu will give you a hangover far worse than real beer. Just be sure to supplement your drinking with water, and maybe choose a cocktail in between here and there.