I have a mixed opinion. Their songs are catchy and fun to listen to. But it's definitely not punk, atleast not what I know as punk (Sex Pistols, Black Flag, old Beastie Boys). Also, I hate the way they look, which brings me to my next point:
I HATE what they started. Thanks to these guys saying they're punk and appealing to the middle class tweens, they created a new annoying group. The scenesters and emos. Not just Green Day, but all the bands like this: My Chemical Romance, AFI, NOFX, Nirvana, etc. All these pop-punk bands spawned these losers. I can't go to the mall without seeing a 12 year old in neon-green spandex, a purple sweater, and a Spider-man purse. Guy or girl. The mall is full of these weirdos and it annoys me. They're not only weird but sketchy. You always hear about people getting stabbed at that mall, West Edmonton Mall, and these are usually the culprits.