To write a good riff is one thing... to play it for 25 minutes because you know the prepubescent crowd is going to eat it up is entirely another.
But this isn't the case - they had self-indulgent songs, it's true, but in general that came out in their incredibly self-indulgent live shows. On records, Dazed and Confused, Whole Lotta Love & other ten-minutes-on-the-LP stoner rock songs were the exception, not the rule.
So their riffs are good, and maybe to some (myself included) at times sound just so goddamned awesome they make my head spin; and no one denies the quality of the drumming... the lyrics are painful at times, self-indulgent live shows, and they probably used drugs and had premarital sex. Still, not close to crap.